Thank you both!
Swwils: I was going to lay the floor in this direction, as this is how I laid it in all the other rooms, and I am using the same flooring throughout the flat, so I thought it might not look right if it runs in a different direction in this one room. The whole room is basically big windows all around, as it's a conservatory, so hopefully it will look rigth!
NutJob: thanks! A door strip / threshold might work, if I can find one that's 2.4m long, very flat (like the one you linked above - they look quite flat), and white. Though these tend to be for normal doors, and come in 90cm length. I definitely want to leave decent expansion gaps, as it's a conservatory and there can be huge changes in temperature (even in one day). The room is not big through, only about 9 m2. But still, better be safe than sorry!
The other thing I was considering was to get a strip that covers the whole UPVC sill, plus the expansion gap - essentially creating a new sill, something like this:
https://www.wickes.co.uk/Wickes-PVCu-White-Cloaking-Profile-95-x-2500mm/p/162625. This is thin enough, so the sliding door would still operate as normal, and I can make it look nice, seal at the ends and the long side where it will meet the door. The skirting would go on top of it too. As long as I can glue it down properly (maybe use UPVC pins too), it could work.