You can order a SIM simply enough through my link here:
https://giffgaff.com/orders/affiliate/nogginphone I think you get a free £5 when you get this and no cost for the SIM card
I joined a month or two ago and initial experience is good however there are things you should be aware of as it came as a surprise to me - sorry if you know all this.
Giff Gaff is an online only service, run by members through a community style forum.
You can earn money back through helping people on the forum and you can ask for help which is usually provided pretty quickly, but......
If you have an issue, you can't just ring someone for help, instead you post a question to an 'agent' and then wait up to 24 hours for a response or them to sort out the problem.
I had a few reasons to get in touch and they always sorted the issue no problem, however, I had to wait a few hours and you don't know when anything is happening, so a bit frustrating if you're used to just shouting at a call centre and getting some action.
A good point however is a very nice Giff Gaff app to monitor your balance.
Also you can buy monthly 'goody bags' containing your preferred service e.g. 300 mins, unlimited texts etc.
The nice thing with this is its completely flexible in that you may know next month will be a quiet one, so you drop to PAYG, but then you want a lot of talk time so get the appropriate service just for that month.
You can set these to recur so it act's like a normal account, or just play around with the services how you want
Once I got used to the fact you get help through the forum, and accustomed to the website which can be hard to navigate initially, I really like it. Coverage is generally good (O2 as said above) and the flexibility is really nice.
Also, when you have a rolling monthly goody bag, you can also store normal PAYG airtime. This can be used as backup in case you run out of minutes or go abroad and need to cover some roaming fee's for example. Through the App, its really easy to see the account balance and I now prefer to other services I've used.
At the very worst, if you don't like it you can drop out easily anytime, especially if you do the PAYG service
I've been on a learning curve with the service so drop me a PM if you need to know anything specifically with the service or using the website.