It's been a while since I've seen a Type F, but IIRC the hex-cap screws are the same as the type M; they are 7/32''. Unlikely you'll find them in England; I've found it harder to get inch tools in Europe than metric tools in the USA.
As Fohat.digs said, it's 7/32 for the M and 9/32 for the F. Yeah Metric bolts and such are not too uncommon here. Even without all the foreign cars, lots of things follow Metric here. Computer case screws for example.
That said, while I can roll down to the local auto parts store and get a 5.0mm nutdriver, they wouldn't have a 5.5mm. They might have a 7/32 but usually not.
Something to watch out for is that there is very little clearance between the hex head and the screw well; ordinary nut drivers, of the sort of thing that come in a kit won't fit.
I do recommend a thinwall ,like you said, but the model F has a little more clearance, so the not-very-thin Impact wrench nutdrivers i have still fit.
For the Model M though I strongly recommend the Wiha picofinish 265P thinwall 5.5mm nutdriver, though my cheap chinese set probably gets more use.