depends on how long and what u are using it for 1.5mm will cover 16 amp no probs if fairly long and a heavy constant load 2.5mm
All the 13a leads I use on site are 2.5mm but that's because they lay better and are less prone to damage.
Try to get an H07 grade cable. It's rubberised, very sturdy and easy to roll up. Infinitely better than cheap PVC type cables
Another vote for H07RN-F. It's what I use unless I really need it to be a specific colour (Yellow for 110V, Blue for 230V), in which case I'll use Arctic. The H07RN-F handles better. Cheapest place I've found for it is https://www.online-cables.co.uk/h07rn-f-heavy-duty-rubber-3-core-58-c.asp.
Echo above, depends on what you are doing with it and how long as 1.25 would be rated for 13A but you are much better of going for 1.5mm H07 as it is just a superior cable and much nicer to use. Don't skimp on the plugs and sockets either, go for permaplug or duraplug. I've often found stage lighting suppliers to be extremely competitive on H07 cable if you need to make a few up and can buy a drum.
