Couple of suggestions.
Why not stay at Exmouth if it is a nicer camp. BUT
On the way there, or on the way back, add an extra couple of nights in a B&B near the Eden Project and Heligan.
Eden project and Heligan are very close together and you could stay at St Austell or somewhere like that.
That way you could do both places in the one trip AND still get your holiday in Exmouth.
I have been to both Eden and Heligan, and I am not sure when your holiday is next year but if it is in the summer (July / August) Eden can get VERY busy and is rather uncomfortable.
Why not consider going to Eden / Heligan at some other time of the year when there are school holidays but it is not so busy.
I realise Brighton is quite drive, but a weeks holiday in that area during Easter or other half term would be more pleasant than fighting it out with the thousands of other visitors in July or August.