The difference is that with bidet, as also with words like ballet, beret, buffet, café, cliché, and debris, in North America the stress falls on the second syllable, while Britain favors the first syllable in each of those words.
Wikipedia calls this “French stress”, and has quite a long list of these. A few of those are wrong, though, like for example négligée, which is stressed on the first syllable in both but which receives secondary stress at the end in North America.
If you mean the difference between the two L’s in the word little, the first one [l] is the same as the Italian L of your acquaintance and is sometimes called a “bright L” or a “clear L”, while the second is an allophone of English sometimes referred to as a “dark L”, written [lˠ] or [ɫ].
The key difference is that the dark version is velarized, which here means that the back of your tongue flattens out and moves up a bit. The normal demo-word for this sound is FULL. If you have ever heard Catalan spoken, you will find that the Catalans often have dark L’s where a Castilian speaker would have a bright one like you do. It probably sounds a bit “swallowed” to your ear.
The exact distribution of these two allophones for phonemic /l/ varies by region, speaker, and syllable position. Some speakers have only one or the other of the two, while others have both. [reference]
Technically speaking, the dark L is:
The velarized alveolar lateral approximant, also known as dark l, is a type of consonantal sound used in some spoken languages. The regular symbol in the International Phonetic Alphabet that represents this sound is ⟨lˠ⟩, though the dedicated letter ⟨ɫ⟩ is perhaps more common.
In contrast, the other L is:
The alveolar lateral approximant, also known as clear l, is a type of consonantal sound used in some spoken languages. The symbol in the International Phonetic Alphabet that represents dental, alveolar, and postalveolar lateral approximants is ⟨l⟩, and the equivalent X-SAMPA symbol is l.
Best Answer
In AmE, writer and rider are generally not distinguished unless the dialect in question happens to exhibit Canadian raising. Note that despite the name, Canadian raising is not exclusively Canadian, and you can find speakers exhibiting it in the US as well. The difference isn't in the "w", though--it's in the first vowel. See the Wikipedia link for details.
In addition, there may be a small difference in the length of the first vowel in AmE. According to Peter Shor's comment, you can find this difference in most American dialects, but as I understand it there isn't generally a large enough difference to reliably distinguish the two.
In BrE, /t/ and /d/ sound different, so writer and rider sound different.