Sounds good, Marine-Tex is a white pigmented 2-part epoxy, maybe you have the equivalent available in UK. Price of a small jar with activator for filling scrapes and gouges and chips isn't terribly expensive so even ebay might fit any budget. This stuff trowels on as a thin putty once activated, feels and absorbs into fibrous cloth much like an oil would then polymerizes during cure to lock everything together into a solid.
You can use acetone to clean up before curing takes place, and a little acetone can be used as a thinner if necessary.
Sanding lightly the area of application and degreasing (acetone/MEK) promotes adhesion thus can make the difference of a permanent repair. Sealing is beneficial due to lose glass fibers absorb water into the laminate eventually leading to delamination, epoxy resin will soak into the wound and seal from moisture. BTW, the 5 minute quick/fast cure epoxies aren't as good at soaking into and penetrating the lose fibers.
I also had some of this type of gelcoat chipping damage in my pump tunnel around the leading edges of the impeller, and gouges on the hull bottom from "sand" bars the previous owner ran the boat in NY harbor. There still was a live mussel caught in the intake grate the day I pulled it home, lol.
As my repair I simply sprayed the exposed areas inside the pump tunnel where gel coat was chipped away using epoxy household appliance paint in a spray can, the type used for repairing washers and dryer appliances, it's liquid as dispensed and absorbs well, into the lose substrate. Using a few heavy coats to stabilize and seal the exposed fibers and reduce chances of further delamination from moisture penetration.
I had a chip in the deck where someone had dropped an anchor, filled this smooth with Marine-Tex for a no-show repair.
I think if it's still a bit rough (not perfectly smooth) inside the tunnel it's not going to impact performance IMO.
I generally use the Marine-Tex (my boat hull color is always white) to fill gouges and chips in the bottom, sand smooth and spray a leveling coat of epoxy spray paint if desired. I've done this many times to my boats and others to pretty them up in spring or fall and seal the substrate, it's not uncommon to get scrapes and gouges over the years.
White is often just white preferably, occasionally egg-shell perhaps, which can present a challange. Maybe the guys with fancy sparkly colored gel coat will go to the trouble of having their gel coat color matched to the factory color or factory matched to the gel coat color?
