I do a fair bit of cashback noodling myself, including TopCashback but wouldn't've particularly thought to look at this.. may v well be handy for a spare (thinking especially of the increasing number of demands for a number for OTPs). Would then use as mentioned above, the top up £5 route & do the minimum to keep the sim active. Cheers for posting!
Re the main topic, just to mention if no-one else has that even after a number hibernates because of no chargeable activity you still have a reasonable margin to call in & have it reinstated - EE for example is 179 days for hibernation but only after 273 will your number & credit be lost.