Assume every tax rate except for the “personal allowance” was increased by 1% to fund free train travel.
Would you be in favour of this?
I've not done the math for this (though did look up some stats for a Scotland specific post, and it seems to be feasible: https://discuss.tchncs.de/post/44818305). So for the UK it may require a lower or higher increase (and to be fair my Scotland "calculations" were very rough and likely entirely wrong). This is more just a question about whether people would be in favour of something like this in general.
Do I get a tax exemption for road construction if I don't drive a car?
It's good for society, do it. We don't need every tax to have a bunch of bureaucracy piled on top of it making it less efficient. Rising tides raise all ships.