UK: Best Prepaid SIM Options for the UK

There are two types of SIMs on 3's PAYG (Pay As You Go) network. Either Data SIMs, or Phone SIMs.
The Phone SIMs have option of two great add-ons, with either the 'All-in-one 10' with calls, text and 500MB or 'All-in-one 20' witheven more calls and text and 'All-you-can-eat'. 
The data SIMs have some good chunks of data, and there is also a pre-loaded option (3GB for 3 months). Best bit about 3 UK is they give free roaming in 15+ other countries with both of the SIMs - Awesome!

If you are heading to the UK “Big Bundle” prepaid SIM card gives you a bundle of minutes, texts and data.
There are two top-up amounts.
£10 = 100 UK mins, 1000 UK texts, 500MB UK web
£15 = 200 UK mins, 2000 UK texts, 1GB UK web
£20 = 400 UK mins, 4000 UK texts, 2GB UK web
Data add-ons available (eg 1GB = £10).
See more detail about the O2 prepaid SIM card here.