Reading between the lines i reckon the mrs has got saggy boobs from having the kids right?
let me tell you something about saggy boobs. they are extemely distressing for the person that they belong to, especially when they have been better. i had a nose job because i hated my nose (it had a bump on it) and it made a massive difference to my confidence so i cannot cuss anyone who would want implants. If it's just coz they are small, that can be distressing too because clothes don't look right and you always feel self conscious about it.
so... give your wife £3.5k. tell your wife she can spend it on her boob job (but suggest a boob reconstruction may be better than implants, as implants need to be re-done every 10 to 15 years and they have to scrape a bit more of the flesh away everytime they do it... so long term it might be better to go for the reconstruction... so an alternative idea she may want to explore).
then keep £1.5k and spend it on a bike.
that way you are both happy.
ps. i'm not being unfair by suggesting wife has £3.5k and you only have £1.5k. i have said this because
(a) 1.5k will buy you a pretty decent which you will be well happy with (i'm sure you can negotiate a £2k bike down to £1.5k if you are offering cash.
(

3.5k will get a pretty decent surgeon. if they cost more she can save up the rest, but make sure she goes somewhere she can trust and has a good reputation and she researches it, and all the options ([type of]implants/reconstruction).
(c) your sex life will improve too. even tho your wife initially gets more, you will still get the benefit of the tits too, whereas it is unlikely she will want to ride your bike. so she should have the extra bit.
ps... i have to be honest... if your wife's boobs are only small in her eyes (i.e. perfectly formed B cup or more and still in good nick) then i would talk her out of it. because if they have to be removed they have to take some away so she will end up as an A cup, and besides... big boobs sag when you get older and particularly with false boobs they start to look really false when you lose weight as an old lady... and as said they have to be re-done every 10 years or so. be honest with yourself, do you think they have sagged and that's why she's upset or are they less than a B cup? if the answer is yes to either question give her the cash, but if they are still in good nick and she's just paranoid, don't.