Series 15 of Top Gear pre-dates the Equality Act (2010), which prohibits businesses from denying based on the sexuality of their customers, by 8 years.

The sign is probably partially to say "no loud drunken hen/stag parties" but also is very likely a not-so-discreet coded message that says "no gay people".

This sort of shit used to be common in Britain until much more recently than people realise....