Here in the UK we are limited to combinations that have been issued
Not for a long time. You can ask DVLA to issue a plate (old style) provided that it has never been issued before and they'll then bung it into auction.
It used to be that they'd issue suffix or prefix plates only. (ABC #A, or A# ABC) but now they'll issue anything at all, including very old # A or A # plates from counties/registration districts that never used to exist. Nice little earner for them.
The kicker is that they won't always tell you when it goes up for auction even if they promise they will, and the process might take years, so you'll either need to be checking every listing or pay a plate dealer a substantial sum to do that for you.
Years ago I enquired about COS 1C, was promised it'd go into auction within the year and that I'd be told about it, only for nothing to happen until it cropped up at auction 6 years later to be bought by a plate dealer for £700. (I'd have had that, as an heirloom) He had it up for sale for £10k for quite some time, dropped to £7k for about 3 years, then a chap with a Sierra Cosworth bought it because if you stuck a screw in the final C it said "COSIE" and that was almost Cossie.
I think it's more fun in the UK as you've got to be more creative. The chap with 2B and NOT 2B. Little Mazdas with MX5 4 MUM and MX5 for DAD. SPF 15 on a 3 litre Austin Healey ragtop etc. Cheeky spacing is fine as the ANPR has no issues with it and it's more memorable for humans. Misrepresenting the letters with screws or silly fonts takes the mickey though!