They probably can, but you need a solicitor to advise you in detail. However, they cannot force a sale without first getting a County Court Judgement & you defaulting on the terms of it, & then they have to go back to Court to get a Court order to sell. It is possible they might have to sell subject to your father's tenancy - i.e. he would still be able to live there, but this raises a lot of other issues, such as level of rent, keeping the property repaired etc.
If they do start a County Court claim you must respond. If you do not accept that you owe the money you must enter a defence. If you do owe it but cannot pay it in full now you must supply your income & expenditure details (form comes with the Court claim) & make a monthly instalment payment offer.
You say the debt is an old one. If it is more than 6 years old & no payment or acknowledgement has been made in the last 6 years then it may be statute barred. This does not mean the debt is written off, but does mean they cannot enforce it in Court & hence would be unable to get an order for sale.