Forgot about this thread until just now. Funnily enough, after all the difficulties trying to do it properly, I ended up going to a farm just down the road and the farmer had no issues at all. So now I have a 300 yard field to chuck arrows down anytime I want. Bootiful.
On the subject of sourcing yew here in the UK, I've found the best way to go about it is find people who sell wood-turning materials. Yew is used very often due to it's colour and sap/heart boundary and if you ask them if they have any bigger bits they haven't yet chopped up they tend to say yes. It's also a very good idea never to mention how hard it is to find. I learned that the hard way, and had to pay too much for some yew logs. Handled carefully, you can either pick them up for free or a fiver if you're friendly enough.
Never had much luck with churchyard yew. It grows very crooked and quite often now the yew trees found in churches are fast-grown landscape yew which is pretty rubbish quality, especially if you're building warbows.