you could try a motor factors local to you, by the time added postage to internet price might not be much in it plus you walk out factors with them no waiting for delivery.
I got the discs and pads for my classic from MM-4x4. Pads were about £10 plus p&p.
Just looked at the P38 ones. Less than £40 for discs and pads. Front or rear.
Don't fit inferior brake pads to any vehicle, it can be a frightening experience. Mintex pads are a good proven make & don't cost the earth though will be more than your £10 chinese import crap.
If your pads still have meat on them anything over 6mm or 7mm will last a long time. A stripdown and clean and a tube of Copperslip grease is a lot cheaper than new pads. also anybody who pays £200.00 for P38 pads needs their bumps feeling. You should be able to get top notch pads (Mintex or Ferodo) at your local garage factors for well under £30.00. But remember new pads on old discs or even new discs require a good bedding in period. A couple of hundred miles at least before any heavy sustained use. Or you risk warping the discs.
The brakes I had for my classic were mintex. Like you say cheap ones are exactly that cheap, and inferior. I would not stake the life of me or the kids on them.