The liner is there solely to protect the frame from scratches. With cables that run along the top of the top tube, the simple act of standing over the bike at stop can cause the cable to rub the frame and leave scratch marks. The liner keeps that from happening but that's it.

Shimano makes an obscure product (most online sites do not carry it) called "housing liner" (http://www.cheapbikeparts360.com/pro...housing-liner/). You might have a better shot at finding it in the UK than the Jagwire product.