Hi - totally wrong end of the country but I sent a 20 yr old polo pony to retirement earlier this year to Cornwall. She is at Gorgeous Grazing ( I asked this forum for feedback and got very positive posts, might be worth searching).
My pony is so far very happy, looks great and I am really really pleased with the decision, which wasn't an easy one. The costs are also very good compared to everywhere else I tried.
I am actually heading down to see her next week and can't wait to give her a big hug and many many packets of polos! http://www.gorgeousgrazing.co.uk/
Not Yorkshire but closer than Cornwall - I do retirement livery at grass from around £140 per calendar month which includes trims and wormers, apart from daily checks and haylage as needed. NE Shropshire.
There's a nice one in Cheshire/Staffordshire I've been to the yard it's very nice http://www.cheshireretirementlivery.co.uk/
Also came across this one by searching but don't know what it's like http://www.retirement-livery.co.uk/ you will also find loads on http://www.*******.co.uk/ just select retirement as a search option