An teresting topic this one - good thread.
I think it self polices fine, one or two so called pro players do sometimes slip in I am sure and there is nothing that APAT can do about that. A poker pro who happens to living in Nottingham might well play an APAT event at DTD for example. I am informed the same thing happens in pool.
For me a pro would be somebody who can make a living out of poker, something that is incredibly hard to do (as opposed to making a nice amount on the side, which is just hard to do). Anybody who can do this would certainly have an edge on an APAT field. It"s all about how you classify a pro. There may be a few APAT players who can, on their day, play to a level equal to somebody who calls themselves a pro but is really just making some nice money on the side. These few can play very well on their day but will have more off days than our "pro" and therefore over a good sample size have a significantly smaller edge on the field.
A true pro who does truely make a living at poker would have to have a large edge on a APAT field, anybody who does not would not be able to do it.
The big pros would have a slight advantage (not a large one at all) but the pros who would play a £75 event would really nt have a big advantage if any at all. The average person really overestimates the skill level of an average pro.
Maybe, or maybe we all underestimate how hard it really is to be good enough to be a pro. I have yet to meet anybody at an APAT event who I am convinced could do it successfully long term.