Normally one way tickets over the Atlantic are expensive but flexible. In other words BA/IAG tries to keep lower cost inflexible tickets to return flights only and this is a hugely important aspect of the airlines' approach to revenue management. So normally if you were on a flexible ticket then moving to another day would only cost a fare difference plus any change fee. Changes could be low cost or zero. However some markets (Point of Sale) are different, and Spain is one of them. This is thanks to the fact that competing airlines such as TAP and RAM do have cheaper trans Atlantic one-way fares, but are restricted fares. IAG uses Level to try and control this to some degree. But from Spain it is possible to find inflexible one-way tickets. In your case there may not be a through fare to Port of Spain too, it may be an add-on fare from MAD to LHR, which in turn could be inflexible. So a lot hinges on the precise fare rules of the ticket you originally bought. Hopefully you retained a record of these fare rules, or they are visible in MMB, to see whether the agent was correct to upgrade the ticket to flexible and then move it or not.