I don't know about the laws in the UK, but here in the US, even if you put up signs, you can still be sued. Putting up signs simply goes to show the court that you were being a responsible beekeeper, and it shifts some of the responsibility for being stung to the recipiant of the sting, which decreases the likihood of a judgement against you. Though I tend to agree that it does increase the odds that someone will try to sue.
If you have liability insurance though, your insurance company should be willing to fight the suit on your behalf, and as long as you have more coverage than any claim being made, you would not be touchable by the plaintiffs. The question is, how much is enough. Well that all depends on the severity of the injury. If a person just get's stung 1-10 times and has no severe reaction, less than 10,000 USD would be sufficient coverage, but now let's say someone that's severely allergic to bees gets stung and dies. In that case, I would want about 4,000,000 USD in coverage. However, in the state that I live in, the courts here would dismiss any such case, as the person who got stung is at least partially responsible for being close enough to the hive to get stung (as long as warnings are posted). That's not the case in all states however. California for example, will figure out the percentage of liability, and split any award by that percentage, so that if the judge decided that a bee sting was worth, say $4,000 and the courts found that the beekeeper was 25% responsible while the plaintiff was 75% responsible, the beekeeper would have to pay the plaintiff $1,000.
Anyway, that's here, not there. Just thought I'd share for the benefit of those here, hopefully someone else knows how it works there.