counting backwards
£550 this month, £275 last month, nothing December, £250 November, £150 October, £125 September, £50 August, £200 July, £75 June, £150 May, £75 April, £50 March – from a full pot.
that’s £1950, which is 3.9% for the year.
Advantage is no tax due, so if all your allowances were used up elsewhere that’s an effective rate of 4.875% I think, or more depending on your tax bracket.
Disadvantage is that it’s not compounding as the account is maxed, so you have to put it somewhere else at a similar rate to effectively get compounding.