As you are not travelling from or to outside the EU, whatever cigarettes you buy will have to be duty paid. But the duty in Spain is a lot less than UK duty.
The official line is that you can bring 2900 Spanish duty paid cigarettes back with you without question.
Any more than that and you will have to somehow show that you will not be selling them on.
For guidance, 200 cigarettes will probably cost you 27 Euros which equates to about �2 per pack of 20.
We sometimes get a cheap flight to spain, never leave the airport, buy 1000 fags and get on the next plane home. It still works out cheaper.