You can order a new sim using this link https://giffgaff.com/orders/mgm
You should recieve your sim in 2 business days (or next business day if ordered before 5.00pm but you should allow upto 7 days during busy times of the year.
You could pop into your local superstore who should sell giffgaff sims. I think they are £1
Once you have your sim then all you need to do is a simple simswap, See this link with all the info you need about performing a simswap https://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084
If you want to arrange hand delivery, it would be useful to know your rough location, but don't post your full address on a public forum.
I'm currently in Bath, by the way.
(Actually, this phone that I'm using claims to be waterproof, but I don't think I want to put it to the test!)