Hi,
You'll get every variation for recommendations. Some networks are better in some areas than others. You could ask some friends who are within the area you are usually in and on different networks for their views, or you could buy a monthly PAYG SIM and try each one out to see which network suits you.
Is your Apple 5c an American sourced handset? if so, check to see which bands it supports in terms of 4G/LTE. The UK uses a different frequency band for 4G/LTE signals. If you think you'll be fine with just 3G bandwidth access, you'll find the handset will work. If you have a UK sourced 5c then disregard the above.
I have 3 SIM cards/plans. EE is one, 3 is another and the last is a PAYG GiffGaff one which uses the O2 network.
Soon, with the recent mergers and acquisitions, the UK will have 3 main network providers - Vodafone, 3 and EE ( I think! - I can never remember who o2 are now or going to be owned by!)
Cheers, DtM! West London & Slough UK!