Are you the sole beneficiary, or is it more complex than that?
In terms of releasing the ISA into the estate accounts, it’s very straight forward, no different to any other bank account really.
One thing to note though, is any solicitor will wait a minimum of 6 months from grant of probate to distribution, it’s best practice in case there are any legal challenges to the will, it also gives them ample time to call in money, and pay outstanding debts from the estate, HMRC declarations, and prepare a final statement of account..
Once the 6 month period has passed from grant of probate there’s no reason for the solicitor to delay distribution though, unless there are complications.
Your solicitor should have probably told you about the ‘6 month rule’ though, to be fair.