I bet that if a group of people were:
1) Motivated and trained
2) Large enough and well equipped enough
3) Not afraid to die
They could take out the electricity supply for all, or huge parts, of London or other major cities for long enough to make those cities uninhabitable.
1 is a pre-requisite
2 isn't as onerous as you might think
3 if 1 is applied, 3 doesn't even need to be a consideration.
There are well known ways to make an HV electrical installation unusable, in fact allied forces used them to knock out power in Iraq - the method used was
supposed to just make breakers trip due to a flashover fault and then allow reset, but in practice it required replacement of all the insulators affected.
Without going into too much detail, a few modified fireworks (and in particular a slow fuse to allow retreat to safe distance) and I reckon an HV line could be effectively knocked out until insulators were changed. Do that to enough lines (which conveniently go for miles and miles in open country) and you'd cripple the grid. If that happened, where do you think the operators would find the numbers of skilled staff and equipment to fix multiple simultaneous faults ?
Even if you failed to come up with a reliable means of creating a 'permanent' fault, apply enough temporary faults to a line and it would still be out of action while the operator set out to inspect it. How many times would they reconnect a line that immediately re-tripped with a flashover before deciding it's no longer a good idea ? It might not take out the power for very long, but it could still create massive problems - and especially generate a huge propaganda storm.
Yes I could see it needing a bit of practice and "research" - but if it's someone else's infrastructure you're buggering about with, who cares about the cost