When I clicked on those links above, only an error message came up.
So here is the southbound schedule that was displayed for Wednesday, March 4:
Inverness, Bus Station 18:55 Perth , Broxden Park and Ride 21:10 21:15 Kinross, Park and Ride 21:35 21:35 Halbeath Interchange, Park & Ride 21:48 21:48 Edinburgh, Edinburgh Bus Station 22:15 Edinburgh, Edinburgh Bus Station 23:00 Newcastle, John Dobson Street 01:35 01:40 Scotch Corner, Motorway Services 02:30 02:30 Wakefield/Woolley Edge, Wakefield/Woolley Edge Southbound Coach Park M1 03:45 03:45 Sheffield, Meadowhall Interchange 04:05 04:05 London, Victoria Coach Station 08:00 London, Victoria Coach Station 10:30 Amiens, Gare Routiere 16:55 16:55 Paris, Porte Maillot Coach park 19:00 Paris, Porte Maillot Coach park 21:30 Limoges, Benedictins Gare Routiere 02:45 02:45 Brive-la-Gaillarde, Centre Commercial Ouest 04:00 04:00 Toulouse, Matabiau Gare Routiere 06:40 06:40 Barcelona, Estacion del Norte 11:55
The Megabus option has a travel time of 40h. Fare is 32 GBP.
There could possibly be different rail options as well. One could be:
07:55 Inverness15:51 London Kings Cross
7 hours 56 minutes
Fare is 37.50 GBP.
London short layover:
17:31 London St. Pancras
20:53 Paris Gare Du Nord
2 hours 22 minutes
Fare is 46.00 GBP.
London longer layover
20:01 London St. Pancras
23:28 Paris Gare Du Nord
2 hours 27 minutes
Fare is 34.50 GBP.
After a transfer to Gare De Lyon
07:13 Paris Gare De Lyon
13:30 Barcelona Sants
6h 17minutes.
Fare is 58.50 GBP.
Seems like the rail option is feasible as well. With 130.50 GBP in total rail fare cost (booking in advance), it is still more than 4 times the bus fare, at the same time, the travel times are shorter, with spending about 16 and a half hours inside rail vehicles, compared to 37.5h in different buses.