Cashback mortgages are often aimed at first time buyers. The lender will offer a lump sum of cash at the start, or sometimes at an agreed point during the term, of your mortgage.
Usually the cash-back is offered as a package of benefits (e.g. linked with a discount) but pure cashback mortgages are not uncommon and whilst some or all of the cashback may have to be repaid upon early repayment of the mortgage, you may be offered a really useful sum of money that could help with the costs of moving. This could be a flat amount, or a percentage of the mortgage loan.