A big pile of "it depends", just the definition of senior engineer varies so much from company to company that comparison is all but meaningless.

I know that at Facebook the ballpark salary for something approximating senior is approx £100k. I think a range between around £60k to £110k is in line with what i've seen on offer for senior (source: i've spent most of this year looking for work, and this is pretty much what i've seen)

It depends far more on the company than anything else. I've seen engineers earning over £120k that wouldn't have been able to get hired as senior at my last role, where the senior salary was in the £70-80k range.

As a (mediocre) Senior engineer at a London startup i'm on 75k per year + up to 20% annual bonus.

FB/Google/Microsoft seem to be in the 80-100k ballpark for senior engineers.

Maybe outside London. I recently left London but took my salary with me. If I needed to get a new job where I am I'd probably be looking at a 40-50% pay cut.

My experience is it is possible earn a lot more than that in the UK as a senior engineer, even outside of London