I was there recently
and the conditions are mostly 4D with a consistent 75% pen (+/-5%)in the HIGHER class joints and some 6D in the LOWER classe joints with 75%-80% pen. ENHC dealer takes all so don't double down or split against a 10 or A. The other rules are as mentioned in the other posts (S17,DAS,D9-11,RSA).
Yes, you must join 24 hours in advance, but you can do this via fax before your trip. You will get letter in the mail upon approval. Mostly all will waive the $1000GBP (around $1800US) initation fee. The will want to see ID (passport or drivers license) and will take your picture. You can get 6 people in on your signature.
I used Hi-Lo. The games are mediocre but the pen is consistent. Everything is regulated by the government. Mostly low roller action even in the swankiest places. Usually $10GBP minimums. The low end joints have more tables open at any one time and are better for wonging oppportunities. The high end are small and James Bondish with maybe only 1 or 2 BJ tables open. All except The casinos on Leicester Square (Hard Rock and Napoleans) have dress codes. Also try Palm Beach Club, Sportmans, and Crockfords. I was told to stay away from the Gala casino chain that is all over the country due to their games being 6D with 66% pen (4/6). The casinos open at 12pm, but the gaming goes from 2pm-6am.
Heat is minimal, but if you bet anything over $50GBP you will get noticed. No comps there--it's against the law--but food, which is excellent for this reason. Don't play too long in one place at any significant bettng level. They seem to be very polite and want to avoid confrontation so they are unlikely to back you off, but I did get half shoed at one place after my 2nd marathon session.
I hope this helps. Good Luck!