The UK doesn't do exit checks. The fences are there to stop people a) trying to enter the UK illegally or b) planting a bomb on the Eurostar.
They CAN do exit checks, so it's not correct to say they don't do them.
I was checked on the 27th December at Heathrow (T3) and I was part of the pre-Eurostar testing that had full exit checks for training, and Eurostar has had them occasionally (but nowhere near enough) since.
Sadly, we seem to be rather reactive than proactive, so if they're doing exit checks it's probably only because there's a risk of someone leaving.
As for stopping people entering illegally, they've got by immigration in Paris, Lille or Brussels, so besides passing customs (if they turn up) there's no reason to jump the fence - you just walk out as normal! Who is there to stop you exactly?
To stop a bomb? Hmm, if they make it hard to get a bomb on a Eurostar train, why isn't a terrorist just putting it on an Intercity train instead? There's only so much damage a single bomb can do anyway - it's not going to take out an 18-car train. I doubt the fence is for that either, even if you say it as if it's an undeniable fact.
