You'll struggle to source any tyres in the sizes suitable for C3's etc from any of the leading European marketed companies.
The BFG's, Coopers, Generals etc are generally available in the UK with the BFG's right at the top of the tree pricewise (the other brands being less expensive).
If you are prepared to accept the fact that these sizes of USA sourced tyres were never historically classified with official speed ratings - and only now are rated with maximums of between 113 mph and 118 mph - and your 'mindset' is of the "well, the UK national speed limit is 70 mph, why would I need tyres that have a higher speed rating"?
The reason why you should perhaps consider a tyre with a speed rating 'matched' to the potential performance of your vehicle is not the fact that you'll ever drive at 120 mph or 140 mph - its because your C3 (depending on the year) is physically capable of those speeds. The stresses placed on tyres are not from just high speeds alone. Cornering speeds, the effects of torque from some of the higher powered engines will possibly have an effect on the longevit, safety and serviceabity of tyres.
Perhaps look at the Vitour Galaxy R1 255/60/15 @ £119.90 + vat. It's rated at 149 mph. Why not ask Forest Gump (Andy Maskery, Essex rep) - he has done a few track and vehicle handling days with these tyres - I'm sure he will share his thoughts.
Pirelli Cinturato CN12 is a 255/60/15 and speed rated at 168 mph - price £369.00 + vat
A race spec' but road legal Avon CR6ZZ 255/60/15 also rated at 149 mph but priced at £314.00 + vat. How good is it?, I don't know - perhaps ask forum regular One Ball in a few weeks when he's done his first track event?
