Yes, a tennis racquet is allowed as hand luggage but it should form part of your hand luggage allowance, ie. one piece of hand luggage and a laptop or handbag.

When a flight is not full we'll obviously give more leeway over hand luggage, but flights are extremely full at the moment and likely to remain so until at least the end of school holidays. If it's a small racket then it'd probably be OK in addition to your hand luggage. However, if it's a bag like Serena Williams carries onto Centre Court, ie. a pro bag, it will probably have to be checked in.

You can check the racquet(s) in free of charge as they're sporting equipment.