Too many unknowns to say for sure. What type of tasks do you use the computer for? Is there any software you use that might not be fully compatible with Apple Silicon Macs on day one?
The iMac that is rumoured to be the first to transition over will be the smaller iMac (rumours suggest a 24" form factor). If you're looking at the larger iMac then you'll almost certainly be waiting to next year on the basis that the new 27" Intel iMac has just been released.
However if we're comparing the new rumoured smaller iMac on Apple Silicon vs. the current larger 27" iMac on Intel, then it's difficult to say. The baseline 27" starts at £1799, whereas the current Intel 21.5" starts from just £1099. So assuming that the rumoured 24" iMac is a compelling purchase (at least more so than the current 21.5" iMac) and starts from the same price or less, then the employer contribution may not actually count for much.
In addition, rumours suggest that the 24" iMac may be released by the end of the year, or it may be in Q1 next year. If it is released this year, then you could still use the contribution towards a new Apple Silicon iMac.
Personally I would wait until October to see what happens. Apple likely won't release a new iMac in November or December based on previous releases, so you should know what to do by the end of October. However, as always, if you need the machine now and it's for business purposes then you should buy it now.