Here is a stab at costing free further/higher education.
University fees can be up to £9,000 per student per year
There were 2.28 million students in higher education providers (excluding further education colleges) in the UK in 2015-2016.
The cost of their fees could total
£9,00 x 2.28 million = £16,560,000,000.
That is more than sixteen and a half billion pounds a year (taking a billion as a million million) If you follow the US in taking a billion as a thousand muillion, it is over 165 billion pounds a year
This number of students seems not to include those in further education colleges. I couldn't find any numbers or costings for those, but they would add considerably to the total. Then there are the mature students taking advantage of "lifetime learning" for the rest of their lives.
If fees were paid, many more students would enroll who are put off at the moment by the cost. That would be a good thing for their education, but the cost to the taxpayer would be astronomical.
Promise anything that does the trick is the watchword.