I've played at the mall a few times, the first was a standard skirmish. it was when they first started, no cap on members it was absolute carnage. Full auto almost everywhere. And since it's not designed as a site, it can become one sided. The second was a mil-sim, semi everywhere with far less numbers that was one of the better games i've played.
If it is a social thing for work, with airsoft. Make sure anywhere you go has a rental system, and book plenty in advance, several places also want deposits so bear that in mind.
For retailers i've used a lot of them. Fire support, land warrior, Xsite airsoft. Zeroin (and second hand through their forums). There are also plenty of HK places like ehobbyasia, ebaybanned, airsoft-club etc. That you can use. Although bear in mind import duty and courier fees. They are all generally the same, maybe a few pound here or there different.