Looking at the weather forecast, it might be OK, it's not wall to wall rain, showery at worst and a lot of places will remain dry - watch the 2 minute video, rather than looking at an app.
www.bbc.co.uk/weather
Bad weather or not, I'd be concerned that a lot of the indoor attractions like the Eden Project or aquarium will be booked up already - check before deciding. What were you planning to do in Plymouth and can you still do that if it's less than optimum conditions?
The weather is a big reason why I'm reluctant to holiday in the UK. Sods law seems to dictate that it's fantastic while you're at work, but any time you're on holiday and want to do things outside, it rains.
But then I'm not averse to having a holiday at home either. It's not having to work or stick to a timetable that's the attraction to me, freedom to go out for the day, sit and read a book, whatever.
I don't think of it as 'staring at the same four walls' and the only attraction of staying somewhere else is so that I'm nearer to different attractions that would make a day trip impractical.
And no, it's not because I have a big posh house, I live in a small 2 bed ex council semi, that most of Mumsnet would be very unimpressed with in terms of location, decor and cleanliness.
Yesterday I went for a drive across the North York moors and stopped to look at the view, look around the little villages, listen to the complete silence and then had lunch and I did think 'I could come here on holiday' but then also thought that I could go there for multiple day trips whenever I wanted and tick off the places I wanted to see more of over time - most of them are only an hour or so away from where I live and not run the risk of awful weather during a booked break.