Some info from a group newbie

If you have any savings and would like the interest gross then register with HSBC for an online saver account. They specify in the terms and conditions that if you become non-UK resident (ie: after you tell the IR you're no longer resident) you can get interest gross after submitting an R105. No tax re-claim necessary.

This is exactly what I have done with First Direct (a member of HSBC) but since the HSBC rate is better (5.75% vs 5.25% AER) I would go for them. Note that these two rates have not yet increased since the BoE hiked borrowing to 5.5% so I expect another rate increase soon.