It seems as though Club Europe semi-flex fares are now the sweet spot in terms of maximising TPs per £.

Did some basic calculations based on the cheapest Club Europe semi-flex fares I could find between LHR and NCL. Caveat, that this is based on a sale fare.

Total fare: £335
Taxes/fees: £88
Eligible spend: £247
Return bonus: 550 TP
Total TP per return: 797
TP per £: 2.379

Number of trips required (rounded up) for:
Gold: 26
Silver: 10
Bronze: 5

Cost to attain:
Gold: 26 x 335 = £8710
Silver: 10 x 335 = £3350
Bronze: 5 x 335 = £1675

If one was to put £1000 into SAF thus achieveing 2k TPs, then the cost based on flights to achieve TP threshold - 2k would be as follows, each achieving a moderate saving of £5:
Gold: 23 x 335 = £7705
Silver: 7 x 335 = £2345
Bronze: 2 x 335 = £670