I assume that you're refering to mainland Spain (or the Ballearic Islands), rather than to the Canery Islands. (Although the Canary Islands are part of Spain, and therefore part of the EU, they have special tax status which means that the 'non-EU' rules apply to importing ciggies from there).
EU law (which still applies during the post-Brexit transition period) allows you to bring back a theoretically unlimited quantity of tobacco products as long as they're for your peronal use. ('Personal use' includes genuine gifts to other people but excludes any imports for which you receive anything in return).
However HM Treasury has set a 'guideline limit', above which customs officials will tend to assume that cigarettes aren't for personal use unless the traveller can convince them otherwise. That limit used to be 3200 cigarettes. Gordon Brown tried to reduce the figure to 800 in 2002 but was forced to back down. However Justine Greenin reintroduced the plan to reduce the limit to 800 in 2011 and, partly because it was by then the same figure used in most other EU countries, was successful in doing so.
So the current guideline figure is 800 cigarettes, as shown here:
https://www.gov.uk/duty-free-goods/arrivals-from-eu-countries
You
might get away with importing 2000 cigarettes but you'd risk losing the lot by trying to do.