I drive weekly between Central London / Portsmouth / Twickenham / Birmingham / wherever needed. About 3000 miles/month.
The services on the M1 - Leicester Forest (?)- seem a fair price.
M3 - Fleet Services - currently around 72.9p/l
Cheapest nationally is Birmingham Star City , Bristol Adsa and Morrisons good prices (Cribbs),
Also check out "WilleyField Farm" or "Potters Poultry" on the A5 between Lutterworth and Hinckley as prices can be very, very good (pcode CV23 0SQ Near Leicester) Call them first to confirm price, but it's 24 x 7 automatic machine.
A34 Peartree OK, less than the LPG stations north on the A34 towards M40.
Shell on A316 southbound near twickenham is 2-3p litre less than anywhere within 40 miles AFAIK (68p/l compared to 71.9-73.9 todays prices). http://goo.gl/maps/muUZb
If using M25 near A1/M1 then Sainsburys Colney are good prices, and free air for tyres.
Also, Near Greenford London - http://goo.gl/maps/fDpQa
good prices. A4020 Uxbridge Road - LPG Accumulator card as well (If still running)