CG67180 - Gifts: hold-over relief: partial claims
You should not accept any claims to hold-over relief which try to limit the amount of relief to a figure of the parties' choice. Such partial claims may be made to try and utilise the donor's otherwise unused annual exempt amount.
If, however, you are dealing with a case where a number of separate assets are transferred so that a separate chargeable gain accrues on each then the parties are free to choose which of those transferred assets are to be the subject of a claim to relief.