I agree, Lancaster doesn't need the RG title is show it's a good uni, you can look different subjects on league tables by different websites and Lancaster is usually in the top 10. The tables are based on research power, research quality, teaching, student satisfaction and employment rate - so Lancaster is doing really good without the title.
However, it's all up to you because this is your future. At first I only wanted to go to RG universities (coming from a family that pretty much wants to show off to relatives :/) but then I realised that RG isn't everything so I actually decided to compare universities and read each modules to see what interests me because I'm the one going to be studying it for 3 years not my family.
Also look into accomodation and living costs. How much would it be living in Cornwall or Lancaster, how's the city life, will there be places you want to go to - nightclubs/bars, restaurants, entertainment. You could read what Exeter/Lancaster students have posted about their experience or go to an open day if you haven't already done.