hi, i dont recommend going via almeria, very time consuming and no cheaper than flying to places closer to the ferry crossings from algeciras. they go so frequently throughout the day to late at night. almeria is something like once a day or twice a week or something. and you end up way over at melilla.
look at airfares with www.flymonarch.com luton to gibraltar - then its only a 45 mins bus for 1.60 euro from la linea (about 5 mins walk from gibraltar airport) to the algeciras bus station which you can then walk 5-10 mins to the algeciras ferry or stay at the bus station and get a bus to tarifa and ferry from there.
if you fly to malaga and you can get cheap flights to there too then its 1 euro by bus to malaga bus station and 11 euro for the 2 hour bus ride to alegicras. or from the malaga airport get on bus that stops outside the aiport doors and leaves every 1-2 hours to marbella then get on the bus from there to algeciras - its the same one that leaves from malaga bus station.
so you have a selection of companies that do cheap flights to malaga.
then you can look up www.atlas-blue.com and pay to get there direct with no stuffing around with connections, bits of time here and there, ferry tickets which still add up to 50 euro return
and get a direct flight for £140-150. you can also do one ways with them which is also a bonus and an alternative.
so i definitely dont recommend almeria or even catching a ferry from malaga - they take too long.
also coming back if returning from tangier then you can save money and time by catching a ferry to tarifa from tangier, then they have a bus included in the price meeting the ferry on arrival to take you to algeciras. you can get off at the terminal and walk to the bus to malaga or you can stay on the bus to the bus station and the bus to la linea and gibraltar is just around the corner.
the other bonus with gibraltar is tax free and very cheap alcohol and perfume and cigarettes if you unfortunately smoke.