To add a little to that, the prefix letters for age identification you mention run back to 1983, previous to that (back to 1963) it was the other way around - suffix letters.You can't just have any layout of letters and numbers, it needs to be of an existing format. AFAIK there are 3 styles:
1 - present/modern style: AB16 ABC (2 letters, then 2 numbers to signify the year, then 3 letters)
2 - prefix style: A1 ABC (1 letter, 1 to 3 numbers, then 3 letters)
3 - Northern Irish style: ABC 123 (3 letters then 3 numbers, but I'm not sure if those are just the plates available, or if that's the rule).
sure, there are other styles of plates out there, but they're ancient ones that can't be "bought new" - you'll have to buy it off someone else, and I doubt a quirky old style plate will be cheap.
Can look through examples and available plates here: http://dvlaregistrations.direct.gov.uk/
Note, not all letters are available, e.g. there's no I or Q.
And, afaik, the only other rule is (apart from nothing rude), you can't have a plate showing a year newer than your car. So, if your car is registered in 2014 then a 15, 65 and 16 plate is out.
Also you can get Q - it denotes a kit car, import, something like that.