I'd say talk to your midwife or GP.
Make sure your insurance is OK and you can get a refund if you have to cancel last minute. I got a nasty scare at my 25w appointment which meant a cancelled holiday & rebooking flights to a wedding. Luckily it was just a scare, but an expensive one. (And one that I am still moaning about to anyone who will listen. :-/)
Also, you may be a lot more uncomfortable at 27 or 29w than 21. I was fine & chipper (& flying around Europe) at 21w. At 27 I have pelvic girdle pain which is relatively mild but makes sitting in the car a pain in the something-close-to-ass.
DVT is more of a risk when you can't move around. How about taking the train part or all the way? I know it's more faff and probably more money, but there's the new direct service to Marseille from London, and the routes through France are easy even if you just do a change in Paris. You can easily get up and walk around on the train, maybe even get a sleeper. Try seat61.com for your destination in Spain as an alternative.
You may also need a fit-to-fly type note for the coach journey on the way back. I know the ferry companies ask for fit-to-travel notes at about the same time as the airlines start to (28w) so you should check that with the holiday company.
A tip for those - I was told by a midwife, that midwives can sign them after a routine visit and usually don't charge. My GP charged £50!