How was the 15mm copper attached to the steel?

The steel was probably an ersatz substitute for 3/4" copper pipe, which
was used until 1971. The 1/2" pipe was the same OD as 15mm tube, hence
the apparent 15mm tube to the rads. There were material shortages from
the end of WW2 until the early 70s and all manner of stuff was used.

You could probably get a joint if you get some 3/4" x 22mm compression
adaptors from a plumbers' merchants, or 3/4" compression olives to fit
in a 22mm compression joint. 3/4" pipe is about 21mm OD (can't find the
micrometer).

You could rip it out and repipe with PEX.