A flexible lease commensurate with financial performance, ie: Sustained loadfactor and sustained Scottish market share, of 2 new 767s would definitely be more dependable and viable than it would be to refurbish old TriStars.
The early generation RB 524s are maintenance intensive, especially sensitive to vibration. These 3 spool motors are not as dependable as the twin spool CF6s on the Diesel 10. I don't believe that those -524s can do more than 2000 hours on the wing.
But because these birds are so cheap to buy, it's possible to keep 2 spare ships in operational readiness at each side of the Atlantic.