Is there any a particular SIM free ("unlocked"?) thing you have in mind?
It really does depend on the service providers in your area and also the number and type of base stations. Ask around and find recommendations. For example in my location I tried O2 (terrible) and T-Mobile (reasonable) before settling on 3, mainly because there are four high quality 3 base stations very near me.
For mobile broadband coverage by provider see here:
Ofcom's base station location map may also be helpful:
Giffgaff has simple tariffs both for phones or tablets/PCs where only data is required.
I use 3's sim only plans, topping up as when I need to; but rolling contracts are available too.