unusual certainly. Likely not English C18 - why? - mostly, I'd suggest because of the colour and shape - English C18 bottles were in what collectors call Bristol colours - deep blue, deep strong greens in several shades and amethyst - and as Andy McConnell says "Early English coloured glass is rare".
The shape here is unlike what we might expect to see from that period - it may well be a shoulder decanter, but there is a lack of smooth wide radiused curve where the neck meets body, and the neck ring, or string ring, is a feature out of place mostly on C18 bottles - they do occur, but very rarely.
The nipple atop the stopper is also not an English feature - at that time - later in the C19 but not mid C18 - something the Victorians were keen on, and similarly with this paler apple green which was something common in second half C19, along with other lurid colours.
Remember too, a bottle from the latter part of the C19 is going to be 130 - 140 years old - and quite capable of showing a lot of wear.
Regret I've not a clue as to origin, though from somewhere in Continental Europe is likely.
When you speak of a similarity to 'export glass from the 18th century' - may we know the origin of this thought, and some more details which will help us to get a better picture.
Sorry I'm unable to agree with you that 'it's of a great age' - but you did ask for someone old to comment, though as Bernard always used to say - I'm not remotely an expert.

Assume you've looked in McConnell?
Hopefully, someone will come along and rubbish my suggestions and tell you exactly where and when it was made.
P.S. any chance please of some details on height - thanks.