The modern option doesn't need to look modern. But by purchasing the modern options that look vintage you can drop your costs substantiall.

Ikea sells quite a lot of galvanized stuff, like lanterns these cute watering cans a lot of different plant pots etc..

If you search galvanized wedding, there are actually a lot of results! Maybe if you have galvanized pieces as accents (a planter full of iced beer, lanterns for tealights or some things like that), then highlight the peach with table runners, cushions on those rocking chairs, in the table numbers, escort cards, flowers etc, it'd tie everything together visually.

I think if you go for an English Country Garden type theme it could be really elegant and beautiful, whilst keeping the homegrown/vintage feel you want. I recommend searching for English tea parties and country fetes for inspiration, as the Pinterest mason jar/burlap thing isn't really so much part of that culture. It's more Peter Rabbit, delicious homemade sponge cakes and cricket. A cottage garden feel could maybe be what you're after, in which case it's something England is famous for.

If high-end is more your thing, than there are definitely a lot of manicured fancy country gardens in manor houses in England, so that may act as inspiration, whilst avoiding the hay bales and tractor imagery conjured by the biggest pinterest trend for barn weddings (although they can be very beautiful too, it sounds like it's not your thing).

British magazines like House and Country, or looking at weddings held in National Trust gardens might fit the bill. If you like what you see I recommend watching Downton Abbey for lots and lots of inspiration.

If you have a Pinterest, search for "Vintage Wedding" or "Garden Wedding" to find the style I think you're after. Go with your gut. I'm sure it'll turn out beautiful!

Decorate for yourselves. Because that is what you are doing. You are shaping an atmosphere for yourselves. Literally nobody is going to notice this or criticize it. They will probably think "Oooh, the space is so pretty!" but unless your family are mostly landscape designers or theatrical scenic artists, it's just not a thing people pay a ton of attention to at weddings.