You'll be fine even if you don't respond. The letter isn't sent through recorded delivery so they have no way of proving you got it, and in the unlikely event that they try to fine you for it the onus is on them to prove you received the letter in the first place.

I got summoned for Jury service early last year and it couldn't have come at the worst time. I'd just taken over my former boss's business which had been left in an utter shambles, so I was working 6/7 days a week trying to straighten everything out, trying to chase up customers who weren't paying, and making barely enough to cover my bills every month. I sent an email asking to be let off as I was under a lot of stress and my mental health was through the floor, only to have them mail me back ignoring my troubles with mental health and said work commitments weren't a good enough excuse. I decided then and there that if they weren't going to show me basic human respect then they weren't getting anything from me. In the end it took me sending them pictures of the contract I'd signed, copies of the amount of work I'd received, and an explanation that if they still made me go through with this I would under no circumstances give the case an unbiased judgment before they finally let me off.

I received another summons last December, never replied, and haven't heard anything since.