You won't be able to do an IMC rating as you don't hold a UK licence to attach the rating to. You could however do the full FAA IR which cost wise would not cost you much more than the IMC and is far more useful.
FAA PPL includes night (you do 3 hrs training at night) so by that token you can actually fly at night straight away. There are a few odd ball things though - the UK requires all night flight to be in accordance with IFR. Now an FAA PPL cannot fly IAW IFR without an IR....People have different interpretations of these rules and I'm sure someone will be along in a bit to expand on this.