Before we get too carried away with Cornwall, the only cheap airline to go anywhere near on flights from the Netherlands is easyjet to Bristol and they only do it at a usable time once a week. Otherwise getting there in connection with a cheap flight will probably involve staying somewhere else in at least one direction.
Presumably your previous Ryanair experience was from Weeze/ Niederhein. Your options with them are Birmingham, Edinburgh or London Stansted.
easyjet go from Amsterdam to Bristol, Doncaster, Edinburgh, Liverpool, and three London airports.
Jet2 go from Amsterdam to Leeds.
KLM seem to go to most UK airports from Amsterdam and can be as cheap as the above if you avoid the business peak times.
All seem to allow 16 year old onwards for unaccompanied travel. On these routes distance doesn't seem to make a lot of difference to price.
As for clubs, don't build your hopes up as most will require photo ID not just looks and many set their own age limits at 21 or 25. Your ID (other than for the flight) would need to be a proper passport not a national ID card to satisfy most checks. The pub problem would not be restricted to alcohol, they may not even allow you in and that could block access to reasonably priced food other than take-aways.
My suggestions are
1. to fly to Leeds or Doncaster and stay in York, which has everything except the beach. You'd need a bus or train ride to Scarborough for that. You stand a good chance of meeting up with similar minded people in a York hostel, probably Australian or American (making you the best English speaker!!)
Or
2. fly to Liverpool and stay in Chester or Southport. Chester has history, Southport has a beach. Both have easy access to big shops and tourist sites in Liverpool and Manchester. And they have some local nightlife, especially at weekends. Chester also gives easy access to the North Wales coast and the Snowdonia national park with it's mountains. From Southport you could reach the Lake District as a day trip. I can't recommend a lone 17 year old girl staying in Liverpool or Manchester overnight.
Or
3. Cardiff. KLM go there. It's not England but they do speak English as the main language (much to the annoyance of the Welsh nationalists!). The airport is close to the city to save money and time. If you watch British TV, Cardiff is the city where they film Doctor Who and Torchwood. It has everything you want, the nearest beach is at Barry Island just outside the city and that's a TV location as well on Gavin&Stacey. Just don't pick a weekend when there is international sport at the Millenium Stadium (normally that doesn't happen in summer).