I play electric guitar but will need to also have drums, bass, keyboards and vocals in the mix.
Budget wise I'm thinking £50-100 but can be flexible if it means getting something that's a lot better
What country do you live in?
That budget is a bit tight for decent CIEMs - how about well-isolating UIEMs? The brainwavz B100 could be a good match and is inexpensive.
I'm unfortunately not really familiar with CIEMs in the <$300 range. For a bit more than £200, the LEAR LCF- (or the LHF-version for better resale value since the LHF has only got removable, customised nozzles but universal shells) AE1d is a model that comes into my mind. It's a single dynamic driver IEM