In general there is no tax on gifts.
Firstly IHT. If this is your mum's first substantial gift within the past seven years, don't worry. If your mum has made substantial gifts to other people or into trust in the past seven years exceeding 325k in total (eg 200k to you and more than 125k elsewhere), I'd get more advice about whether IHT might be due and who would pay it in the event of your mum dying within 7 years.
The first 325k there is no IHT to pay, above that the recipient of the gift may need to pay the IHT within 6 months of death, not the estate. If you are the sole beneficiary in your mum's will after Probate is granted you would then receive her estate.
Deprivation of assets - if your mum later was in council funded care and the council believed she reasonably expected to need care at the time the gift was made, then they may seek to recover the funds. For example if your mum was frail and in declining health currently and it was looking like she may need care. If she is in good health and independent currently that is unlikely to be a concern.
Also I presume your mum has other assets that she is keeping, so if she needed to pay for care then these would be used first, it would only be if her assets were exhausted that the council might look into previous gifts.
If you are worried that the gift might become subject to IHT or deprivation of assets I would still accept it, but use it with the knowledge that it is possible it could be reclaimed (partial for IHT, possibly in total if deemed deprivation of assets for care costs and her other assets are exhausted). If you paid off your mortgage, you could always remortgage in the small event you need to return the money. Don't put it into a risky investment or spend it on holidays!