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Check Shell's Web Site - that'll tell you all the Optimax stations on your route (although I found it was missing a load in Cornwall on one trip I made)
I used a couple on the M6.....Southbound - Corley (near Birmingham) and Charnock Richard services is Shell as well (a few others I passed on the M6 were also Shell)
There is one at the Stansted junction (8??) on the M11 as well I believe.
If you were desperate there is one a couple of miles off the M25 at the Waltham Abbey junction (26??)
The Shell website really is the best way to find them all though.