Useful guidance from Trading Standsrds :
At this challenging time it is likely that either your customers or you may need to cancel contracts you have entered into as a direct result of the coronavirus epidemic and the legal restrictions that have been brought in.
Legally, a contract may be 'frustrated' if an unforeseen event occurs after the contract is formed, which makes it impossible to perform. Coronavirus would fall into this category, so if you need to cancel a contract, for example because you need to self-isolate or your customer does, this could make the contract impossible to perform. Remember, this is not due to the action or fault of either party, so there should be no penalty where a contract has to be terminated in these circumstances.
As always, the best thing to do is to speak to your customer and come to some kind of agreement. It may well be that you can delay the contract until such time when the restrictions are relaxed thus enabling the contract to be completed when there is no risk of either party contracting or spreading the coronavirus
There is a lot of information available online - https://www.gov.uk/government/publications/guidance-to-employers-and-businesses-about-covid-19