or ask me
yes it is OK
make sure that the money can be raised under your articles and it perfectly OK to say that it is to be used for a certain defined subsets
raise it and minute it at a meeting of the trustees
actually it would be OK to have a partitioned account
BUT
if money has been raised for X ( which is allowable under your articles ) and used for Y - then I am amazed that an outraged party did not contact the Charity Commission as it is clear breach of trust
Partitioned money must be collected for that purpose - altho you could use some of your general money to augment it - and used for that purpose
and the money raised for a purpose has to be used for that purpose - the general rule is that if it is NOT it is to be returned to the donor.
( or used for a very like purpose under the cy-pres rule)
Cy pres rule is not very well explained here:
https://en.wikipedia.org/wiki/Cy-pr%C3%A8s_doctrine
as it mainly concerns american law)
so yeah a separate acct would be sensible
(I have to say I am shocked to read that money was raised for X and someone came along and said, no Y is better)