How can my sister in South Africa invest in UK?

Arn5

Intenaxx not the cheapest as they have custody fees on top of dealing fees - they are not as cheap as https://www.keytrade.lu or their Belgian parent https://www.keytrade.be where you can get a EUR bank account as well. No GBP account though.

Fidelity offer a money market fund where you can park the money but your sister will need a bank account to move money around first.

But First
Working on the strategy and then fill in the bits needed afterwards eg money goes to ZA, stays in UK, goes offshore (not UK or ZA) etc until you are happy.

DYOR pays off in the end as you will understand better than anyone what you want and how it should work.

Post away later with the questions to fill int he bits.