Okey dokey....the reason why you are having problems finding tickets or why they are way too expensive is because you are looking over such a long timeframe. Your best bet would be to buy a one way ticket to London with someone like Easyjet and then a cheap charter to POP from London.
It's very rare to find a one way charter ticket to the DR however you do find them from time to time. So it's likely that you will have to book a one or two week return flight and just use the outbound leg. These tickets are cheap - I had a look for you and there is a flight leaving Gatwick on 17th May for ?227 (
www.airflights.co.uk).
Then when you want to come home in Dec you can buy a one way flight back to London from the DR - these flights you can only book when you are there and nearer the time. You may even be able to get a direct flight to Madrid.
I believe that this will be your cheapest option - only other option is to pay through the nose and go scheduled for EUR 2000+
Good luck and let us know how you go
Daydream