A single-level dip in cleric can get you three things: the Inquisition domain with its +4 to dispel checks, Turn Undead that can power Divine Defiance, and the ability to activate a prayer bead of karma for a straight +4 bonus to CL.
Ur-priest does get you the last two things, but it’s difficult to enter and you need two levels instead of just one to get Rebuke Undead from it.
Anyway, Divine Defiance is a feat in Fiendish Codex II: Tyrants of the Nine Hells, and it lets you burn a Turn Undead usage to attempt a counterspell as an immediate action. Obviously valuable.
In general, the cloistered cleric variant is a great 1st level to take for a lot of people: you get 4×(6+Int) skills, much better than you would as a warlock, you get all of the above, another domain, Knowledge Devotion from Complete Champion, and you get identify as a 1st-level divine spell, i.e. you don’t need the arcane material component for it. The Undeath domain can get you Extra Turning if you want it, or you can use the other domain to get all manner of excellent things. Check out the Dipping Cleric Handbook for more ideas.
Divine Defiance requires “divine caster level 3rd,” which warlock doesn’t qualify for. The solution to that is Practiced Spellcaster (cleric) from Complete Mage; that will get your cleric caster level (but not any other aspect of cleric spellcasting) up to 5th to meet the requirement. You will also most likely want Practiced Spellcaster (warlock) to get back the CL lost when dipping cleric, as that would otherwise hurt your dispel checks.
The eldritch disciple prestige class from Complete Mage could progress your cleric spellcasting along with your warlock invocations, but it’s not that great and it would require you to take another two levels of cleric to get the 2nd-level divine spells it requires. Not worth it. Shenanigans can get you in without those levels of cleric, and that might be worth it, but if you want to go that route I recommend just asking your DM to adjust the class’s requirements to begin with, rather than get into shenanigans. For example, eldritch disciple expects a Cleric 3/Warlock 1 entry, and then at 1st level gives you invocations only (so you are effectively a Cleric 3/Warlock 2 at 5th level) and from then on progresses both. Maybe your DM would be willing to flip it, allowing you to enter as a Cleric 1/Warlock 3 and then progressing spells only at 1st (so you are effectively Cleric 2/Warlock 3).
The other prestige class worth mentioning for any warlock is chameleon from Races of Destiny. The second-level bonus feat can be changed every day, which means it can be any invocation you need that day (by making it Extra Invocation), or if you get far enough, an Item Creation feat to go with your Imbue Item class feature. This makes Cloistered Cleric 1/Warlock 9/Chameleon 2 a pretty strong build for you, and when you hit Cleric 1/Warlock 12/Chameleon 2 at 15th, you’ll be exceptionally versatile.
I will mention, however, that I have done a counterspelling warlock quite similar to the one I describe above, and while it works more or less, it’s pretty boring. If you are really interested in a high-power spin on the idea of ruining other casters’ day, what you really want is a Black Tactica build: use the war weaver prestige class to buff all of your allies as a move action at the start of combat, and then spend your actions in combat readying to target anyone who starts casting a spell. You don’t ready a counterspell, however: you ready the most reliable single-target nuke you have. In addition to hitting the spelcaster hard, which is painful, this also forces a Concentration check with DC 10 + the damage dealt. It’s trivial to pump the damage well beyond what anyone can hope to hit with the skill check, which means they will lose the spell.
This is a very high-power build that may actually make your game less fun, as your enemies start using similar tactics and combat becomes something of a deadlock, everyone readying nukes against the others, and whoever dares to move first still not getting anything done.
Assuming the PC has successfully rolled below an 11 on the d20 to cast a spell, the spell takes their action on this turn. If it successfully dispels the spell, all effects of the spell are also removed.
An affected target's speed is halved,
This ends as soon as slow is dispelled. Since movement says you can move up to your speed, you are no longer impinged by this effect and any movement you might have left using your full speed can be used.
it takes a -2 penalty to AC and Dexterity saving throws.
This ends as soon as the spell effect ends.
it can't use reactions.
This ends as soon as the spell effect ends. Note that it doesn't say the PC doesn't have reactions, just that they can't be used while under the effect.
On its turn, it can use either an action or a bonus action, not both.
The PC has used an action on its turn to cast dispel magic. Assuming the spell is successful and it dispels slow, and the PC has a bonus action to make, the PC can now take that bonus action.
If they didn't roll below 11 then they are still affected by the slow until their next turn when everything stated above applies. In between these turns they are still casting the Dispel Magic which requires concentration: if that concentration is broken before their action on the next turn the Dispel Magic is lost and they remain slowed.
Best Answer
In your case, Slow dispels Haste when the wizard casts it. No check is required.
Normally, only certain spells, like dispel magic can be used to get rid of an effect after casting. If you ready an action to counterspell, you can automatically cancel an enemy spell, with no check required, by using your readied action, and expending the same spell as the one being cast (or another same-or-higher-level spell of the same school, with the Improved Counterspelling feat). When counterspelling this way, a Spellcraft (DC 15+spell level) is first required to identify the spell being cast. Dispel magic and similar spells can also be used to counterspell, requiring a caster level check as listed in the spell's description.
Some spells have a specific notation in their descriptions that an opposite spell counters and dispels it. In those specific cases, the opposite spell can be used after the fact, and no check is required. Casting the opposite spell will negate the effect of the original.
Ex:
(PF264; emphasis mine).
Edit: Updated the counterspelling description to mention the Spellcraft check, etc.