you can only pay into a pension when you have earned income.
You can pay in 2880 which gets upped to 3600, up to age 75. Last I heard.
If you take it out the next year (which they could object to but probably wouldn't bother), you don't pay tax on the first 25% so make a small profit if there's zero growth.
(3600 x 0.25) + (3600 x 0.75 x 0.80) = 3060.
Less 2880 = £180 which is 6.25% on your money which is slightly better than the building soc.
You can put your money which is in the SIPP straight into a money market fund whcih would have gained at about the same rate as the bldg soc, 5.2% year to date
as well, = 11.45% = £324 up.
If you'd been vaguely sensible with the investment in a sipp for the last year you'd have made +30%=4680 (Or a bit more adventurous, x5=18000, say). Call that G for growth so for 30% G = 1.3.
You get 1.25x uplift when you put it in, and pay 20% tax on 75% of it when you take it out.
So if you use the pension, the £M becomes M.G x 1.25 ((0.25) + (0.75 x 0.8)) = M.G. 1.0625
PLus say 5.2% in the money mkt gives M.G. 1.1125
If you'd put it into an ISA, which would be another possible with a small amount of extra cash to invest, you wouldn't get the uplift but you would get the money out tax free.
So at the end of the year you would have M.G x 1.052
The difference is there, and would remain whatever return you got.