A good minimum standard BS6658-85 Type A helmet will be fine.
Either open or closed face is acceptable the choice is personal
These should be stickered on the back or labeled inside which should not be rmoved, painted over or covered up in any way.

As Jake said they are currently no strict checks in place, but as insurance companys are getting more and more nervous (not your insurance company - the tracks liability insurance) then if your buying now it makes sense to buy one that will be accepted when the rule complying will be more strictly enforced.

HTH