[HarryT]

Well, for example, I've gone for O2, and they offer three different plans:

1 day's access for £2, with a 500MB data limit.
1GB/month for £10.
3GB/month for £15.

Vodafone are much the same, with:

1GB/month for £10
3GB/month for £15
5GB/month for £25.

While "3" offer:

1GB/month for £7.50
10GB month for £15

(which is clearly better for heavy users).

Orange are perhaps best for occasional users, with:

5p/MB "pay as you go", capped at £40/month.
1 day for £2, with 200MB data.
1 week for £7.50, with 1GB data
3GB/month for £15
10GB/month for £25

All the monthly plans are "rolling" 1 month contracts, which can be stopped (and restarted) at any time.

In addition, with their monthly plans, Orange give you unlimited access to "BT Openzone" WiFi hotspots (of which there are lots), while O2 give you unlimited access to both "BT Openzone" and "The Cloud" hotspots. WiFi use doesn't count against your data limits.

So, as you see, there are lots of choices!