advice please re visiting elderly mother in tier 3

Hello, At present where I live is in tier 1- my 89 year old mum has alzheimers and lives at home with my brother in a tier 3 town. He's mid 60's and is still going to work every day with carers going in 4 time a day. Since lockdown Mum has not been able to access the daycare centre for obvs reasons and has deteriorated in all aspects greatly. Consequently my brother and I have been discussing residential care- and he'd like me to go up and help him sort it. My question is- is it wise to do so and am I allowed to? Its a 300 mile trip up there passing through mainly tier 3 towns on the way. Also if I did go would I need to isolate on my return? (I'm an early 60's overweight gran, helping out with grandchildren who live close by.' school pick-ups etc). TIA