It is, but the way it works is that in order to be able register the change in title there needs to be an undertaking from the plumbers [lawyer] that on receipt of £x they will provide a signed form to MissD’s sellers’ solicitor to enable the restriction to be removed, and possibly (though this isn’t strictly required, just usual) an undertaking from MissD’s sellers’ solicitor that on receipt of the completion monies they will send £x to the plumbers.
These cannot be done without £x being specified, so it is needed.
Without the first of the above undertakings MissD’s sellers’ solicitors cannot (be sure that they will be able to) register the change of title (ie fulfill the contract), so they cannot exchange (or at least sign, which is what exchange means in practical terms).
Edit: it’s not that anyway! (See above). But that’s how it would work