The seiko is a great shout. However, I'd be looking for an skx, now discontinued, for around £300, or mint used for £200 ish. Plenty still on ebay. Money well spent, these will only increase in value imo@Johnny Mars Bar
You'll have look to the Japanese to get a decent mechanical watch in that price range, that even most watch collectors would nod approvingly of.
Since you mention nato strap, I assume that you're looking for a diver or a field watch/sports watch.
I'd suggest you consider the Seiko 5 Sports series. Several dial colours and strap types available. Examples:
Seiko 5 Sports Blue Ref. SBSA001
Seiko 5 Sports - Field Collection Black Ref. SBSA111
A dressier alternative, a bit more pricey:
Citizen Automatic Blue Ref. NB1050-59L
Tons of reviews on YouTube etc.
(btw, I'm not affiliated with Gnomon watches in any way. The watches I suggested are available from lots of other vendors as well)