In short my suggestion is to introduce a new Tourist Plan (Visitors to UK) Sim card which is only valid for one month but can be kept active by topping up if required. If not topped up it expires. Once expired it cannot be activated with a top up. This way the telephone number can be reused. I would recommend the card having unlimited calls and text with 3Gb of data.
Also, I would suggest pushing the selling of these sims at airports, sea ports because this is where you need the card the most on arrival to the UK. I had to track down the T-Mobile shop the next day before I could get "connected". You could also consider sending out sims to travellers prior to their arrival in the UK so they are up and running on arrival.
What would be great is if you were able to provide an add-on for Giffgaff users for when they are abroad too but would you be able to offer that outside of the O2 World wide affiliates, if there is such a thing?
I hope this has been explained clearly.
The idea came about whilst in America recently. Prior to travelling I spent ages researching which sim cards were out there and I found there are next to none specifically for travellers apart from really expensive ones and which you had to pre-order.
The best and most convenient one I could find was the T-Mobile Tourist Plan sim card which you had to purchase from the T-Mobile store in America. Much like my suggestion above it was valid for one month and expired automatically. The "T-Mobile plan is a 3-week plan, gives you 1,000 minutes of talk, unlimited text and data, with up to 2GB of 4G LTE data for $30", which is around £24, which I am sure you can beat. Here is the link to the T-Mobile site https://prepaid.t-mobile.com/prepaid-international-tourist-plan
The only thing I would suggest is offering 3Gb of data as 2Gb is very tight if you need it everyday to get around, but then again top ups from Tesco, Asda, etc could solve that.