Trying to find a way to say no to get together

Can't understand your reluctance to say no. You're not lying about your DH's poor health, and you also have health issues with your night-time loo trips.. although you obviously don't need to spell this out. The earlier you tell them the less likely they are to waste money catering for people who won't be attending. Alternatively, if you feel you must go, make it clear that you'll need to leave early in order to drive home safely.

I would go but leave early.
If I didn't want to drive both ways on the same day you could stay the night before locally to them.

I would juts say you're not up to it, wish them Happy Birthday and send them something nice.

Just say neither of you are well enough to travel, tell them you hope they have a great time. and leave it at that. Keep it simple and straightforward.

I agree with fancythat. If you say before the event that you will be leaving at 7pm (which will give you plenty of time to get home before it gets dark) and explain your reasons, you can meet your needs whilst also having what I’m sure will be an enjoyable afternoon out. It’s only an hour away, so if your husband needs to leave earlier because he isn’t feeling well, then that’s still doable.
I find it’s too easy to say no to events that take a bit of effort, but I think we have to be very wary of stopping doing things too soon in our lives, because they become more of an effort as we age and then we genuinely can’t do them.

I would go. And leave when you need to.

Say you are reaching the stage where you can no longer face the long drive when it's getting dark. Suggest you meet up half way for lunch somewhere soon. I'm sure they will understand.

"Thank you so much for your lovely invitation. We are so sorry but we are not able to make it this time as we have both been under the weather recently and need to rest in the afternoon. Perhaps you could visit us some other time which we would look forward to. Hope you have a lovely afternoon and once again, we are really sorry. "

Find an hotel or B&B not too far from where they live, check in before the bbq and return after the bbq.
Simply say you didn’t want to put anyone to any trouble and you wanted to visit a certain place or attraction and make a weekend of it.

I think honesty is always the best policy - so just say thank you for the invite but your husband just doesn’t feel up to going because of health reasons and that you don’t like driving in the dark , they are not excuses it’s how you really feel.
I’ve had to turn down invitations because I don’t feel well enough to go and people always understand. As it’s close family think I’d prefer an honest phone call rather than an unpersonalised text message.
Happy birthday for today 💐

On another occasion if your husband and his brother would like to meet up still, and it's not possible for you to host them in your flat, could they travel to you and all have lunch in a local restaurant instead?

Good replies ladies. Thanks.

Just say that you wish you were both fit enough but you really aren't.
The truth in fact.

thank you for asking us, but we are not up to it.
hope you all have a nice time.

Thanks for the invite but we won't be coming, hope it goes well.

By text is easier and sooner rather than later or it will play on your mind.

If you are asked for a reason, tell the truth, these days you don't enjoy socialising.

They won't insist you go and now they know your socialising days are over will not ask again.

One short awkward text or call and its all over.

Thankyou for your replies. I know you are all right about being honest. I don't like lying to people. My DH doesn't drive and I am slightly arthritic so driving for an hour is uncomfortable for me. We live in a second floor studio flat so hosting family is out of the question for us so it's always us having to visit others. We will just have to bite the bullet and say no. Thankyou for your Happy Birthday wishes. xx

Thank you for the invite
We have other plans so we will take a raincheck
We will call you later in July to coordinate a visit at a more convenient time for everybody

Express regret as that may in part placate your hosts, and explain your husband isn’t well enough to attend.

If your DH is not in the best of health and doesn't want to go then I don't see why you shouldn't simply say that he doesn't feel up to it.

Not being in the best of health is not an excuse - it's a valid reason for turning down an invitation. The fact you've previously used this reason doesn't make it any less legitimate. I would just be upfront and tell your BIL that your DH just doesn't feel well enough for the journey or the visit.

As you get older and things start to go wrong health wise it's often the case that you need the comfort and security of your own home. And, life is too short to fill it with doing things you don't want to do!

Just tell the truth - as near as damn it in a polite but firm way... rather than look for another 'reason' or 'excuse' not to go.

... and - happy birthday for tomorrow, which is almost here! .

Why can’t you just say your husband isn’t in the best of health and you don’t want to travel. Surely they would understand if he’s poorly. And remember it’s an invitation not a summons. Happy birthday for tomorrow 🎉

Why not be honest and say your health precludes you from attending? It’s a fair reason.