I'm not operational these days, and not involved in UK/European ATC at the moment so I'm not up to speed with the details.
But the UK has had an almost unique implementation of some ICAO SARPs for years - these included (the oxymoron) providing air traffic control outside CAS and, more recently, calling all of the stuff that happens outside CAS as Flight Information Service and claiming to comply with SARPs. I think the latter changes happened after an ICAO audit which highlighted the odd UK system. The UK's approach will probably be overtaken by EU regulations, which trump UK law, which will be more aligned with the SARPs. The first EU reg that really affects operations is SERA - you can see it
here - this will replace the UK's Rules of the Air Regulations. The regs came into effect on 4 December last year but States had the option to defer implementation for two years which I think the UK decided to do.
The CAA has been publishing bulletins on the EU regs etc for a while - they'll be on the website somewhere and might be good background if any of this stuff comes up during the interview.
And on the FIS thing, I think the MATS Pt 1 bit on FIS is a very cut down version of the other CAP so they will probably say the same thing but 774 will have more detail that will probably be very relevant at HIAL airports.