Welcome to the Alice in Wonderland world of airline revenue management. I think it was Willy Walsh who said : "If customers understand airfares, we've got it wrong."
From the airlines' perspective there are a number of reasons why the type of apparent anomaly that you have highlighted make sense. There is naturally a conflict of interest between the airlines' RM systems and the customers' requirement to obtain the lowest fare.
In this case, although the airline would discourage it, you can go ahead and book the two separate journeys, but you must appreciate that your ALC-LGW-ALC ticket and your LGW-TPA-LGW form two separate and unrelated contracts. The airline assumes no responsibility if you miss a connection in either direction, although on the outward that seems unlikely. You will also not be able to check your luggage through, nor benefit from a (possibly) greater baggage allowance on the through ticket.
For £230 I think most people would happily accept that. If anything goes wrong though, you are on your own.