Loo stops M25 S Mimms / before Reading M4- anyone help please?

[GreatGatsbys]

Anyone with local knowledge? Bit of a long shot this but...
Heading from South Mimms to M4. Have a health problem sometimes and may need the loo before Reading services. It's only an hour but sometimes that's too long.

Looking at maps, looks as if it is possible to turn off for Colbrook and there is a BP garage /M&S food on the bypass. Can't be sure it's still there!

Is there anywhere else where it's easy to turn off the M25 or M4 and get back on going in the right direction where there is anywhere to stop for the loo?

If all else fails, when you get to the M25/M4 junction turn towards London instead and go to Heston services.

What I can't remember is if that is only on one side of the road and therefore if it will be a pain to turn back around to go towards Reading, (ie whether you'd have to go even further into London) but it's worth bearing in mind in case there aren't any other suggestions.

There's an Asda - actually, several shops, I can give you a list - between J6 and J7 of the M4 (only about a mile between those junctions) and a Sainsbury's just off J7. I can give directions!