The stuff I have seen at B&Q and my local garden centre
doesn't looking very strong and looks very thin. Any recommendations where
to get this online?

I did the same as you ... raised beds, membrane between ... although I used
bark chips as a cover rather than gravel.

However, in another part of my garden, my 'beach' consists of membrane
covered with 4 tons of beach shingle. Note this is nice, smooth rounded
stuff as opposed to small, sharp shards of gravel.

I used the cheap 'n cheerful stuff from the local DIY shop and haven't had
any problems with either cover type.

Best,

- h