No - the cable rating required on a 30 or 32A ring is only 20A, so (in
principle) a 20A junction box would be OK.

Obviously I should have said radial circuit.

5A junction boxes are very common. i.e.

http://www.screwfix.com/app/sfd/cat/...18802&ts=49771

Is it permitted to use these on a B6A lighting circuit?

Christian.