The cost of (sometimes compulsory) purchasing land, properties and all the legal costs that go with that heavily distorts the cost of new railways in the UK compared with most European countries. The cost of the seemingly endless enquiries and consultations, usually followed by some kind of Civil Action, increases the security and administration costs, Parliamentary costs are not negligible to get Acts of Parliament passed. The discovery of Historical Sites cause delays to projects, upping the costs and lastly, but not least, there is always the discovery of unexploded ordnance - mostly just suspect and not real - that also brings everything to a stop which always costs.
It's very difficult to get a direct comparison. Best you can do is to compare the actual Construction cost - without all the rest - between different countries.