The trip would be beneficial for you, your mum's and your brother's wellbeing - that sounds pretty essential to me. The proposed restrictions are (at the moment, at least) to reduce contact rather than stop it entirely. I think you'd be perfectly justified in your trip. If the weather's nice, would finding a nice quiet spot in a park somewhere for a picnic and natter be a workable idea? The chances are that cafes and restaurants will be closed by next month.
This seems to me a well thought out solution which I will suggest the other parties involved adopt.
I've not spoken to them yet so have no idea of their view on the restrictions, but I'm sure they will both want to do the right socially responsible thing.
We usually meet at one of those pub/restaurant chains attached to a Premier Inn.
Hopefully it will still be open in some capacity.
As you say, it wouldn't matter if food wasn't available, but access to some shelter would be good as my mother will not want to sit out.
Worse way we could sit in one car for a short while and eat a snack.
Another thought is a motorway service station, as we meet not far off the M1.
The toilets and somewhere to sit should still be available even if the corona restrictions are biting harder by then.