An FN actioned sporting rifle is certainly "worth doing" IF you can do it yourself or are willing to pay a professional. There are parts, tools and skills required for the job. Assembling an AR15 out of commodity parts with a few hand tools is feasible (in the USA as long as it remains legal here) but a Mauser is a different proposition.
Let's assume you have the action in hand and want to fit a barrel first.
Border Barrels in Scotland is the only barrelmaker in the British Isles that I know of:
http://www.border-barrels.com/
They get 388 pounds sterling for a cut rifled barrel blank, assuming you have the lathe and tooling to contour, thread, chamber, headspace and install it. If you don't, it is 577 sterling for a fitted barrel and another 112 for proofing, which I expect is mandatory there.
Their less expensive button rifled Archer brand barrel is "only" 216 but the labor to get it on the action does not change so a fitted barrel is 385 and the proof test still to be done at 112.
Perhaps you can find a shop which fits less expensive Continental or US barrels. I once saw a website for a small operator there but do not have a link.
Any road, with a barrel in place, you should then consider sights. Is the FN in question drilled and tapped for scope mounts or is it a military action which will require not only that work but bending the bolt handle and changing the safety to clear?
Or will you use open or receiver sights? Those must be mounted to the barrel and/or the action themselves.
Next think of stocking up. Will you carve a nice stock out of a blank of fine old walnut? Or bolt the barrelled action into a plastic affair? Does the FN action include bottom metal; the trigger guard, magazine box, and floorplate?
Finish is required. The action and bottom metal should be blacked, along with a chrome moly barrel. If a stainless barrel is selected, it can be bead blasted to cut glare and will have a fair appearance next to a blacked action. Or the whole lot can be coated with a modern high technology heat cured polymeric resin... which I call by an old name, paint.
A wood stock has its own finish, there is a thread here about the World's Finest French Polish that you can study to see what is involved.
And that is just what I recall being required. There is probably more, as I am not a gunsmith, professional or amateur.
Best luck.
