I think they can (I’m an insurance underwriter for my sins but a niche commercial product so a little rusty on personal lines) but depends on circumstance.
The insurer has to put you back in the position you were in before the loss and within the term, conditions and limits of the policy.
So, you have an Whatever Bike, 2007 model of average condition which gets nicked. The insurer could if they wanted just post an equivalent bike to you. They have fulfilled their obligations.
If your bike is more bespoke, they could if they wanted, phone the frame builder for example, work out a deal with them and sort it that way.
This is usually too much effort for them so they send cash or vouchers. If vouchers are sufficient to put you back in the position you were before the loss (that is you could go to an shop and buy an equivalent bike, or in this case buy an unspecified bike of £300 value) then yes, they have every right to insist on vouchers.
Cuts down on fraud somewhat (and note to OP I’m not for one moment suggesting your claim is fraudulent-its just an observation).
Cheers
Danny B