Thinking that renting in London is impossible without previous credit or rental history in the country is a popular misconception. Yes, many landlords/agents will ask for UK and/or rental history, but, whilst it poses a challenge, there’s other alternatives you can use for referencing purposes instead of UK Credit/Rental History.
Some landlords/agents will be happy to proceed with using references from international landlords. To carry out financial checks, they may also be OK with seeing account balances from an international account. All you need to do, is ask if they accept this at the start of the process.
When lacking certain documents that proof your ability to afford a rental, getting a UK Guarantor may be your best bet. If you have family or friends in the UK, ask if they can be your guarantor. Alternative, there are companies who offer the service to find you a Guarantor.
When all fails, negotiate with payment terms. There’s nothing that proves your affordability more than making an upfront payment. Offer to pay 3 to 6 months in advance to compensate for the lack of local references. Many landlords are OK with this solution.