Apparently Ben Nevis is not Britain's tallest mountain - I've been reliably informed (*) that there is one a full 760 cubits taller lurking around
(*) reliable = a decent news source 
I am baffled! The 5 highest mountains in Britain are (according to Wiki)....
1 Ben Nevis 1345 metres
2 Càrn Eige 1183 metres
3 Snowdon 1085 metres
4 Carrauntoohil 1038 metres (Ireland)
5 Sgùrr Alasdair 992 metres
So three things immediately sprang to my devious mind...1. Either someone has surreptitiously added 163 metres to the second placed
Càrn Eige under cover of darkness or...
2. You read about it in the Daily Mail...
3. Or your question is very very sneakily worded. I noticed you asked
"How tall is Britain's tallest mountain?" rather than you asking
"How tall is the tallest mountain in Britain?"So, if the mountain is
British but not necessarily
in Britain...then this might mean this taller mountain of yours is craftily hidden
under the sea, somewhere off the coast of Britain.
All of which makes me think it can only be the massive sub-sea mountain oddly called Anton Dohrn which, at 1,700 metres tall, makes it almost a quarter taller than the stump of Ben Nevis.
Am I right or am I right?
PS Why is it called Anton Dohrn? He was a German zoologist. What's wrong with a British name?