Let me help you here - I'm one of the folks who works on the grant funding - in the case of two canals (Cotswold & Droitwich) I was the guy handling the money for HLF, and two more (Rochdale and Huddersfield) I had a responsibility for seeing that the restoration was in accordance with the grant terms, which in turn depended on who had given the grant - I was part of a team monitoring for English Partnerships and had to scratch my head very hard when upon abolition of EP the scheme was handed to NWRDA but without enough budget to finish the job. That's one of the reasons the shallows through Miles Platting (?) was only partially excavated - me (and others) deciding that so long as a navigable corridor was created that "would do" for fulfilling the grant purposes.
Grants ALWAYS have a term limitation, as in, the grant contract has a stipulation that the grant purpose must be maintained for a given period once the works are complete. If land purchase is involved in the project, it's usually 80 years. The only way to wriggle out of this would be to argue that, say, navigation wasn't the purpose of the grant but arose as a result of the grant works that are otherwise being maintained - such an approach is unlikely to succeed. The alternative is to pay the grant back - this is on a sliding scale but may also be index linked, so closing a canal 25 years into a grant funded term of 80 years might mean paying 55/80ths back but that would be adjusted for inflation... inflation since 2000 is approx 80%, so closing a canal restored with a grant, before the grant term expires, can be pretty expensive.
BW tried to stop running trips into Standedge Tunnel as they were losing money, but the boats etc were grant funded (this time 30 years) and repaying the grant would cost more than they were losing.