There is a good deal on Groupon at the moment from London taxi app mytaxi. For £5, new and existing customers can get £20 of mytaxi credit.
The website description is not very well written. The deal is not just for new users. I bought a voucher on Friday and applied it to my existing and heavily used mytaxi account and it processed OK.
The small print states that the credit expires on 31st July 2018 (when you redeem it actually says 1st August) and is limited to one code per person. You can, however, buy a further 10 as gifts.
The code is available for immediate use once you have paid. The credit is used in full against your next ride so be careful – you won’t get the full £20 of value if you use it for a short trip.