I currently use GiffGaff. I would say that, unless you are going to regularly use the connection ie. purchase a monthly gigabag etc and chomp through most of your allowance in 30 days, then they are a more expensive option (per what pcoventry76 has already said regarding 20p for initial (up-to) 20Mb, and then 2p/MB afterwards, per day).
For example, a basic O2 simplicity "NO ALLOWANCE" SIM with no pre-paid bolt on will automatically claw £1 (and no more than this) off your balance for a single day of usage giving you up to 50MB for that day (which is cheaper). You can optionally purchase more webdaily 50MB bolt-ons (£1 a pop) on that same day to increase this in 50MB increments as needed. Monthly bolt-ons are again cheaper (they do 30 day half gig for £6 and 1 gig for £10).
The Virgin Mobile offering (again, per pcoventry76) is probably the best (£1 a day for unlimited usage) but reading up on that, they piggy-back on the EE network, and due to various complaints I've read about them, I'm not sure how reliable or good that network is. Plus I don't particularly have much time for EE (personal grievance, I won't spill the details here).
The ISP you are with (A&A) do mobile sims but its probably a bit OTT for what you are wanting. £2 a month gives you the SIM and then you literally do pay per MB, and get billed accordingly. A&A piggy-back on the 3 network, and you get a real (no carrier grade NAT antics) IPv4 address for your connection too.
If you do decide to go with GiffGaff, make sure you do not tether or use any tablet/hotspot device with a non-gigabag unlimited goodybag otherwise they will immediately block you. Unlimited goodybags are only allowed on non-tethered phones and non-tablet/hotspot devices.
Edited by deleted (Mon 28-Apr-14 14:18:25)