The basic pay isn't much different for an FR first officer doing 800 hrs even with the £7000 deduction for the first year. I think I worked out there's less than a grand in it. And then the Thomson job comes with the benefits of a pension, loss of licence insurance, food, holiday etc.
Yes, the Thomson job is partly part-time but does that matter if you're paid a salary? Let's face it, in reality working for FR is also part-time and un-paid over the winter.
I suspect many FR pilots close to command upgrade would forgo the massive extra stress of being a Captain at the low fuel airline and move to Thomson.
This is especially so when you consider that FR skippers on BRK contracts will have the expense of having to commute/live away from home, fund their own pensions in full, get chased by more than one tax authority, and pay out fines when their 200 hr FO screws up a MAD SID.
And expansion stops next year so their future in FR does not look that rosey.