John, here in Western Canada we used to buy soft iron wire in hardware stores by asking for “baler wire”. It comes in 3.1/2 lb. spools. The wire is 0.060 in. diameter. Looking now I find the modern name is Rebar Tie Wire, used for tying the reinforcing rods together in concrete forms.
Should be available in Home Despot or similar in the UK.
HTH, Mike