A new tool has been developed by two European pilots who are also expert programmers and mathematicians
The main support thread for it is here
It is totally free [:)] As a comparison, Rocketroute charges GBP 260/year for their package which does Eurocontrol routing.
As with Eurocontrol routings generally, and in UK airspace especially, some knowledge of the IFR system is needed because e.g. it is possible get a route validated by the Brussels computer at some ridiculously low level like 3000ft which goes right through a load of major airports, so you do need to know what you are doing.
The router (which is a pure website; no “app” needed) has been recently updated to work better on most mobile phone clients.