We have used Clickstay/Rentalsystems (previously Villarenters) for 11 years, quite successfully, but it is difficult getting bookings as a newly listed property, from a standing start, when you are up against properties with several customer ratings. Typically the first page brings up properties with 20, 30, 40, or more 5Star ratings. So newcomers with none will be on the 50th page or so (20 per page typically) so very difficult to get noticed. This also applies to Trip Advisor and some other websites. So, while it would be worth being as exposed as you can afford to be (i.e owners direct charge an annual fee while Clickstay charge commission per booking), local advertising in UK could work. Air BNB is active in some places but not generally well known enough by Brits aiming for Turkey. It looks like Facebook will become a major player so well worth pursuing which is where we've shifted a lot of emphasis.
Joy Lettings are very prominent but we haven't tried them so can't comment on their offering.
There is no guaranteed portal and much will depend on how much recent events in the Med will influence holiday destination choices next year.
Best of luck.