In the UK you can give any amount of money to someone as a gift without the recipient having to pay any tax (income tax, capital gains tax etc) on the gift as given. However, the gift is treated as a PET (Potentially Exempt Transfer) meaning that if you die within 7 years of having given the gift, it may be subject to inheritance tax.
There are a few scenarios that don't apply to you where it would not be subject to IHT (inheritance tax) even if you died the day after making the gift - for example, if you were gifting it to your spouse, or to your children under the age of 18 for their education.
In your scenario, all or part of it - there's a taper arrangement over time - would be treated as part of your estate and would be liable for IHT according to the normal IHT thresholds, rules etc.
There's a good summary here of the thresholds and rules provided by Which? (the Consumers' Association).
Note, I've answered the question as asked, but as the question relates to student loan debt specifically I would urge you to check whether this is the most efficient use of the money. While the student loan debt does incur interest, your children will only make repayments on it at a %age of their earnings over a threshold and any debt not paid off after a certain amount of time will be written off.
What this means for people who don't earn much during their career is that they never pay off anywhere near the full amount of the debt, so the interest rate on the debt is almost irrelevant. So depending on the likely careers they will pursue and the earning potential of those careers, it may be better to invest the money elsewhere or just gift it to them for whatever they want to do with it (that kind of amount would make a very nice downpayment on a house!) rather than specifically using it to pay off student loans.
More information on this is here.