Believe me the 15 hours is not a lot to get used to:
1) The european airspace (especially UK)
2) The standard required
I had an FAA single engine IR and found the CAA multi-engine IR to be quite the step up. Mind you since you have commercial multi-experience, it should be a bit easier than in my case (no multi experience until i started CPL/IR). I ended up doing 25 hours instead of 15, because wanted to be sure I got a first time pass (it worked).
An earlier post compared the standards of the multi-IR to the FAA ATPL and I would agree with that. You won't get away with a wobbly inbound track in the hold like you would for an FAA IR!
One of the main challenges of the airspace is the back and forth transitions between controlled and uncontrolled airspace. Fun moments like doing the engine failure on the missed approach of the ILS, being given a couple of vectors, then suddenly "you are now outside of controlled airspace, goodbye". Up you to figure exactly where you are, how you're going to get to the next airport and who to contact to get radar advisories (flight following)... while still on one engine trying to get a clapped out Seneca to climb. Ah the memories...
The 170A is a "pre-exam" that is run by a qualified instructor (not examiner). It's a full rehearsal of the IR, two hours long. It's not just for practice, the instructor has to sign you off so you can take the 170 (the IR). That can also be challenging, since your 170A instructor will be different from the instructor who trained you. And as much as flight schools claim that standards are strictly enforced amongst their instructor team... It's just not true! I had to re-take my 170A after some re-training because my initial instructor was new to the school and not quite up to standards. Imagine how pleased I was...
But hey, at the end of the day, I'm sitting fat dumb and happy in a 737 so it was all worth it!
P