Creating duplicate line set distance away from a polyline in QGIS?

Surely possible. I presume, UK coastline is a closed linestring, or consists of n closed linestrings or multilinestrings as well. If so I recommend to convert it to polygons (or to one multipolygon, if preferred) using Polygonize tool from the processing toolbox, and then apply a 30 km buffer to the resulting (multi)polygon(s) (processing toolbox: fixed distance buffer).

Hint: If the coastline does not consist of closed linestrings, you should first edit your coastline in order to make sure, that it does.

The processing toolbox can be accessed under menu Processing > Toolbox, or Ctrl-Alt-T.

In the processing toolbox you can use v.parallel in the GRASS commands

Another tool for the same purpose than v.parallel is in

Processing toolbox - GDAL/OGR - OGR Geoprocessing - Single sided buffers (and offset lines) for lines.