Okay, let's see......
Most of the booster kits are 1/144th scale, so take a 1/100th F-105 (there was one in the Ben Hobby/Tamiya range, but it's not been re-released yet), replace the wings & intakes with 1/72nd Starfighter wings, fit a smaller cockpit canopy, then put a pod on each wingtip. The idea is that the original jet nozzle is now a rocket motor for space flight, and the tip pods conceal small jet engines for cruise-to-landing, with jettisonable nose and tail cones. the overall configuration is a bit like a scaled-up Sud-Est Trident.
Any British equivalent would be hamstrung by the unsuitability of Britain as a launch site (too far north), so we'd probably go for air-launch, with something like an uber-Vulcan carrying a fighter derived from an SR.177 or Avro 720 with a tandem boost pack.
Make the date late enough, and you can use Concorde as a basis for the carrier instead. Here's a quick'n'dirty (but expensive) way to make the carrier: start with two 1/144th Concordes and one 1/72nd one. Graft the two 1/144th fuselages together nose to tip, fit the 1/72nd wings, then put all eight 1/144th engines under them. Blank all the windows off because it's just a flying fuel tank. Now add the space fighter of your choice.....