18k gold will almost certainly give it an immediate value of around £500+. Whether the watch is worth more will depend on the quality of the movement and the quality of the maker.

The hallmarks will give you an assay office and a year. I can't tell from the photos, but there may well be a (case) maker's mark as well.

If you can find a case maker and a date and a movement maker, then an internet search may well give you more info about probable worth.