To get any detail on OpenCPN in the UK you will need to buy some
charts, other countries provide them for free, but not the Hydrographic Office.
The Visit My Harbour
charts are £23 for a year and then if used for real
navigation then you should update them. However, if its just
route planning then no need for this.
https://www.visitmyharbour.com/
The alternative is to use the only line
Navionics chart viewer which has a simple route planner:
https://webapp.navionics.com/?lang=e...&key=kt~tHzb_F
However, the chart viewer isn't up to date as the Open Seamap for some features, if that is a concern.