There are multiple points to make here.

1. The fact that you or I can afford £70 doesn't mean everyone can.
2. Elections can be called at short notice. Getting ID on short notice is even more expensive (can confirm since I did exactly that last year).
3. The proposed free workarounds are still more inconvenient for those on lower income.

None of this would be a problem if there wasn't a correlation between difficulty of obtaining ID and voting trends but unfortunately it's true.

Voter fraud does exist in the UK but it's really not a big deal.

In an ideal world there wouldn't be any fraud but you have to look at it from a pragmatic perspective of "how can we get the most representative result?", rather than "we need to completely eliminate election fraud".

The problem with voter ID is that there's high indication that requiring ID will bias the results more than voter impersonation did/does in the first place.