Two comments about that routing.
Firstly it would be rejected today, since the DCT SITET-GWC breaks the rules relating to DCTs within UK airspace.
Secondly, routes such as that are not "airways" routings as such. It is a prime example of a lazy-mans route in which no attempt is made to file along accepted IFR routings; instead a route is constructed which passes the syntax checker in Brussels but achieves little else. This kind of routing creates work for somebody figuring out where they actually can send you without causing trouble.
The Standard Route Documents (SRDs) published by each of the Eurocontrol member states makes worthwhile (if boring) reading when the subject of routes comes up. You can find the UK SRD under the main documents menu on the AIS site. This provides the "expected" routes from each IFR airport in the UK to each other airport and all UK FIR exit points.
Time spent taking a look at that (and its foreign equivalents) will give you a much better idea of what CFMU wants you to file, as well as ensuring that your filed routing is accepted.
2D