I was supposed to be in London this week and got refunds for my theater tickets because the shows were cancelled. One was issued automatically, the other I had to contact the theater box office to request a refund instead of a credit towards a future performance. London theaters are now closed through at least 28 June, so you should be able to get either a credit or refund for any theater tickets before then. You’ll want to check websites for every provider that you bought any kind of advance tickets with, they all have different cancellation policies right now and those policies continue to change as closure dates are extended further (which may mean you are better off waiting to cancel some things until closer to your trip dates).
One thing to note that seems to have surprised a lot of people is that exchange rates have changed rather dramatically recently. Unless you live in the U.K. and use Pounds as your local currency, the refund you get once it posts to your credit card may be lower than what you paid because your refund isn’t worth as much in U.S. dollars (or other currency) as it was when you made the purchase.