Kev,
If I have paid attention correctly (still reading Hoke's book and this site), I believe the AP you are referring to is what people commonly refer to as Acid Peroxide, which is Muriatic (also called hydrochloric) Acid mixed with 3% Hydrogen Peroxide.
Not being from the UK, I am unable to tell you what stores sell those supplies, but would think they could be found at the local hardware stores (for the acid) and the local pharmacy or grocery store (for the peroxide).
Find a post by LazerSteve, and check out his Guided Tour link. It is really helping me out. I can't vouch for Hoke's book personally, as I am still reading it, but that seems like step 2 from everything I've seen (as Harold points out).
Following LazerSteve's link will eventually get you here:
http://goldrefiningforum.com/phpBB3/viewtopic.php?t=44
The first post has a good link you should follow that kind of tells what chemicals are what, what they are used for, and possible places to find them.
Research all of this yourself. I am still new to this myself, and I am learning just like you. I have no practical experience at this. That being said, I have always been of the opinion that you should be thoroughly well read on a subject before attempting said practice. Much like the “measure twice, cut once” mantra of the carpenter, it’s better to be safe than sorry, and much of the chemicals that these processes deal with can be very dangerous.
Thanks,
Greg
P.S. That was a much easier to read post. Thank you for the effort.