Learn English – How to write out numbers in compliance with British usage

You probably don't want to do it at all. Noninteger numbers are best written in their decimal form (1.5, 5.0). If they can be expressed as simple fractions, you can do that instead: “one and a half” for 1.5.

If you absolutely want to write it out, then you'll have to do it the way they are spelt: “one point five”, “five point zero”. You can use oh instead of zero (“five point oh”, “oh point five”) to reflect an informal spoken style.

In the UK, we always put the "and" in after the hundreds in plain numbers. Commas appear to be put after every "power of 1000" term ("thousand", "million", "billion" and so on); I'm not sure whether that's a requirement or just a good idea. However, not even the most knee-jerk believer in Oxford Commas would put a comma after the hundreds. So it is:

One thousand, two hundred and ten.

In the US, they seem to always leave the "and" out, but use commas in the same way:

One thousand, two hundred ten.

There is another alternative, though. With a number like this between one and two thousand, in the UK at least we might still talk about it hundreds:

Twelve hundred and ten.

This is very much dependent on context and personal preference, but it does happen so you shouldn't be surprised by it.