The problem is that the more that gates are forced, the weaker they become. When I did gatelines (not on LU, I should add), the 'problematic' stations had gates so weak that they'd give out with very little resistance. The wide aisle gates were even worse - one station had it away from the main gateline and it was commonly known among staff as the free exit.
I used to challenge people coming in, but would never do such a thing alone or at a 'rough' station. That said, the risk of anything other than a mouthful off the unruly passenger was relatively low. For staff in London, the risk is far higher and the people doing it are an awful lot more brazen. A load of people bashed their way into Wembley Central a few months back and all the staff member on duty did was laugh and take a picture. Clearly, this was some attempt to build up intelligence of who was doing such things. I don't blame him for not doing anything else.
As for people leaving a station, if they're going to jump over and run off, that was their prerogative. Passengers might comment 'what are you going to do about it' but what was I going to do? Run after them?
The only thing that ever made a difference was a security or police presence, or a lot of staff at once. I once took part in a revenue sting and a bloke did an about-turn after walking towards the exit and came straight up to me offering his details. In his words "better not try and hop over, I can see you've got the coppers there".
As for making gates sturdier or taller, you'd have to do it for all of the stations (and run them as close as you can to all of the time) to prevent people getting in. I've had a passenger try and turn on me when they realised they couldn't kick a gate open (to get out) as easily as they thought they could. Staff certainly won't want to be stuck on a station with someone who's going to do whatever it take to escape.