I think the animus against Blair is based mainly on a sense of betrayal by someone most people here started by liking and trusting. Robert Browning's poem "The Lost Leader" (about William Wordsworth) expresses it pretty well. And the last line fits in here rather well.:

Just for a handful of silver he left us,
       Just for a riband to stick in his coat –
Found the one gift of which fortune bereft us,
       Lost all the others she lets us devote;
They, with the gold to give, doled him out silver,
       So much was theirs who so little allowed:
How all our copper had gone for his service!
       Rags – were they purple, his heart had been proud!
We that had loved him so, followed him, honoured him,
       Lived in his mild and magnificent eye,
Learned his great language, caught his clear accents,
       Made him our pattern to live and to die!
Shakespeare was of us, Milton was for us,
       Burns, Shelley, were with us – they watch from their graves!
He alone breaks from the van and the freemen,
       – He alone sinks to the rear and the slaves!

We shall march prospering – not through his presence;
       Songs may inspirit us, – not from his lyre;
Deeds will be done, – while he boasts his quiescence,
       Still bidding crouch whom the rest bade aspire:
Blot out his name, then, record one lost soul more,
       One task more declined, one more footpath untrod,
One more devils'-triumph and sorrow for angels,
       One wrong more to man, one more insult to God!
Life's night begins: let him never come back to us!
       There would be doubt, hesitation and pain,
Forced praise on our part – the glimmer of twilight,
       Never glad confident morning again!
Best fight on well, for we taught him – strike gallantly,
       Menace our heart ere we master his own;
Then let him receive the new knowledge and wait us,
       Pardoned in heaven, the first by the throne!

If he'd become a Catholic before, as well as the problem Joe mentioned (which could probably have been got round) there'd have been political complications. Every policy would have been liable to be interpreted in the media as him either being subservient to or rebellious against Rome.

And there could well have been another political price to pay. It's still true to say that a lot of people in Britain who would be unwilling to express hostility to any other religion don't have any such reluctance when it comes to Catholicism.