If you used the Right to Buy scheme to buy your home, you likely received a discount. The council keeps a legal claim on this discount for 5 years.
If you want to switch mortgage lenders or borrow more money against your home during the first 5 years, your new lender will need to secure the loan against your property. The lender will want their claim on your property to come before the council's claim.
To make this happen, your lender needs to ask for a 'deed of postponement'. If the council agrees, they will move their claim to third place, letting your lender take second place (your main mortgage is always first).
The council will only agree to this if you are using the money for home improvements or switching mortgage providers.