I would like to top up online but I don't have a British account

[Kalwas]

I have just arrived in the UK and bought the O2 SIM card. I wanted to top it up online but it does not allow to use any of my foreign cards and I don't have a British account yet. Is there anything I could do to top up my card or any other place where I can try to buy a top-up? An O2 branch is too far away from me. 

[Enlli]

With the first top up using a voucher is recommended 

Go to any O2 shop or anywhere you see the green top-up logo and ask for an O2 top-up. You can pay by cash or card. Then call 4444 free from your mobile, or 03456 062 277 from a landline, enter the number on your receipt and follow the instructions. Top up between £10 and £50. Your voucher will expire after twelve months.

[gmarkj]

Other ways to top up are in the guide below @Kalwas 

Guide How to Top Up (PAYG)