UK banks are ridiculous and they're hastening their own demise to challenger banks with attitudes likes this. My bank (Barclays) suspended my account and blocked my cards after attempting to make a large payment. It was £2000 to the Student Loans Company. My account was a graduate account. All of the clever machine learning anti-fraud algorithms in the world couldn't figure that one out. Go figure.
Why would you make a voluntary payment to SLC (assuming it was voluntary as most repayments are through PAYE)? That is quite an unusual thing to do.
That only works if the rules stay the same.
I say it's a fair gamble that they won't and that many will wish they paid them off earlier.