When do they arrive OP? Do you have time to get a bolt put on your bedroom door, see if you can alter the shower height [poss not even an option if the bath is under the eaves]. Sharing an en suite is tedious but older people often don't shower daily and you may find that you don't have them traipsing through all the time.
DH has said his mum will 'want to run our household'
You have two choices - you can view it as a holiday from all things domestic for 5 weeks and take a step back, and simply put your foot down that you want to prepare weaning food for DC1; or you can make it clear to your husband that he needs to support you in not being bullied in your own home. There are no winners - in the second scenario you will spend 5 weeks running around catering and cleaning while they spend time with your DC.
Mini-break - yes definitely. Somewhere with more space and something to do/change of scene might be a very good idea. Somewhere like Oxford, Bath, Cambridge, Edinburgh, York where they can be off entertaining themselves. Not somewhere deeply rural. Hire a car unless yours is huge. 4 adults plus car seat, baggage and travel cot etc is a ton of stuff. It's the UK so it's not like you can just pack a bikini and a sarong in a large handbag 
Just laugh at your DH when he suggests that you can send out your DC1 with a sippy cup of cooling breastmilk. That comment alone just suggests to me that he is being a dickhead and is also probably a bit fed up with you having a stroke about the visit [sorry].
With the best will in the world, 6mo babies aren't that interesting. You don't have to chase after them, they can't do much at soft play, etc. They do like to be held and entertained a lot which with 70+ yr old grandparents sounds ideal to be honest. I saw quite a few baffled visiting grandparents coming to stuff like baby sensory etc around that age. I doubt very much they will want to come to a swimming pool with you !
One option which worked very well for me with my parents [who keep a completely different timetable and are generally pain in the ass guests] is to write up your typical routine so they know that you are heading out by 9.30am every morning to do something. That you will be back by 12 to give lunch, bf and put your baby down for a snooze. That 3-5 is the magic window to hold their DGD and ooh and ah, or to take him out for a walk post feed and give you an hour to yourself.
Watch and observe and hold your horses. I got a lot of this sort of "my mum's raised X kids" from DH in the early days. Then he witnessed her about to take our PFB out in 30 degree heat in the middle of the day with no hood up on the pram, sunscreen or sun hat on, water or changing bag. He's a lot more wary about assuming that nothing has been forgotten now.