Maybe we should.
Statistics-wise, let's assume that the repartition of potential combating she people is the same.
Terf Island + their occipied territories' population is 69 millions people.
A conservative estimate of the trans part of the population is about 1%.
World population is 8B, which would make a ratio of 80/69 if favor of trans people.
Considering that a quick search on military doctrine yields the amount of 10 soldiers per 1000 inhabitants, 20 in the case of active insurgency, the numbers are there.
If we expand to LGBTQIA+ people in general, I don't have a lot of stats but polling in the US gives 9%, which I'll round to 10% because I suck at math and it's 2AM. That brings that number to 800/69.
Then, there's the question of armaments. Reliance on the enemies of the UK proved quite effective the last time they faced a civil war so this seems doable.
Understanding how the UK treated the population they colonized as a frame of reference for how they should consider the idea of others colonizing them, it is clear they are clearly not afraid enough.
Did I spend 10min typing this just out of spite for terf Island? Yes. Was it worth it? No.