I think Facebook killed it but http://www.buildthewoods.co.uk/ is worth a look for inspiration.
You are best off getting in touch with and visiting one of the amazing bmx trail spots around the M25 for a look and talking to people directly for building tips, or better still help them build in order to learn more quickly.
A weekend trip to any of the following trail spots would do it: Chertsey, Leatherhead, Epsom, Wisley, Brockham, Winchester. There are many others.
There are trails in Basingstoke and Maidenhead too but I’m not sure on the exact locations or people running them.
Finally, damn you as I would love to be in your position! Check out C&K Contractors in case you have megabucks and want someone to build something perfect for you. They are the bmxers who built the olympic track.
P.S. Grass will not ride fast but nor will it be there for long if you ride often. Don’t fill your jumps with rubbish like logs as it’s a headache when you inevitably move them to make them bigger. Hire a whaker plate for the compacting or use a moped to run up the lips/landings if you can’t be bothered with spadework.
P.P.S. I would drive up to give you a hand as I love building trails more than anything but am abroad for work until December.