Are you going to be sailing this winter?
Hope you have a very warm drysuit
On to your questions:
1-Can you explain the concept of " Freeride " please.
"Free ride" boards and rigs are an "industry inspired" term for gear that's the next level up from entry level or beginner gear, but not specific to one discipline (like racing, slalom, or wave specific gear).
It's the gear that basically does everything and works in a wide variety of conditions for an even wider variety of sailor skills.
2-Are there specific types of sail I should avoid or try to buy?
I think the "free race" type sails are the best for sailors who want to advance quickly and are looking for good performance, easy rigging, and durability.
There are several "types" of sails but they fall into two basic designs based on whether or not they have camber inducers.
Camber inducers are small plastic "internal fairing" devices that fit on the front ends of the battens to "fair" the battens to the mast in a smooth and streamlined fashion so the flow around the front of the mast extends right onto the leeward (convex) side of the sail.
For your "freeride" progressing beginner type sailing you do not really need any cambers.
The other basic design is the camberless (no camber inducers) type and and the sail makers achieve (when your sail is fully loaded by the wind) very nearly the same "fairing" of the batten to mast interface by carefully tapering the battens and seam shaping in the luff sleeve.
So, my suggestion would be a couple of camless "free race" type sails
aboout 1.5-2.0 different in size to give you a good overall range.
For example if you get a 7.5 m2 as your first "larger" sail, look for a similar sail in 6.0-6.5 m2 for use in higher winds.
3-Have you any recommendations on make of sails or is this personal preference? I have been using a Tushingham sail to learn with.
Since you already have some experience with Tushingham, you may want to stay with that brand. I've sailed them in years past (I use Sailworks currently as they sponsor the "A Taste of Windsurfing" tour and the Starboard/Sailworks Demo Tour that I do here in the USA) and they are really very good sails and you can get good service (not that you will ever need it) from Tushingham there in the UK.
I'd suggest the Tush "T'Bird T3 line as the most appropriate for you to advance both your sailing and rigging skills.
It's a "no-cam design that's very easy to rig, and has a huge tuning range (similar to the Sailworks Retro sails I'm using).
Hope this helps,