Skiathos' runway is currently being expanded and should mean from next summer most narrow bodies operating from JSI to the UK, can fly non-stop. So I wouldn't guarantee it will be operated by a 757.
TCX have stops scheduled in Kavala for return flights from JSI in S14 with A321's at EMA, LGW and STN. So it would appear what ever the runway extension is, TCX still won't be operating A321's non-stop as the schedules currently stand. TCX fly MAN and NCL non-stop because they have 757's.
I only assumed that TOM must be using a 757 at LTN in order to fly from JSI non-stop?