What is the easiest and cheapest way to send money to the UK?

My wife's 86 year old mother lives in Scotland and needs some financial assistance to get by each month, so we want to help her out. We transferred some cash to her bank via FNB's international payment option, but the fees were pretty harsh, and it was only a small amount to test.

Is there an affordable way to send a fixed amount monthly, or maybe a lump sum annually?

My wife and I are both British citizens with permanent residence in RSA. We have lived here 30 years and any money we send will be from our savings (so already taxed if that matters).

Thanks in advance.