You can slow down a fixed-wheel bike with back pressure on the pedals, but to use one legally on the road in the UK and many other countries it must have a front brake. That requires a fork drilled to accept a sidepull – or Dia Compe’s clamp-on alternative that fits directly to the fork legs.
There are caveats. The fork has to be steel not aluminium or carbon fibre, and the fork legs need to be retro narrow (25mm) for the clamps to fit. Even then, we had to ovalise our clamps with pliers to mount them to an old Coventry Eagle.
In use, braking performance is surprisingly good. It’s a proper brake, not a legal figleaf. You can feel but not hear some muted brake judder, yet you stop securely and in good time. The brake drop, from its own brake bridge, is 43-57mm and the brake weighs 345g. The only question is: why not simply spend your £50 on a new drilled fork instead?