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Ahhh I wanted to BE Jill in her wonderful Chatton pony world, despite the fact that she used to get maybe just 2 things for Christmas things like a fountain pen and a hankerchief.
I rebought the series a couple years ago and the following things stuck in my mind although they never seemed weird when i read them as a child!
- Every adult man was a Captain or a Major and carried a shooting stick.
- People could afford a couple of lessons a week at the local riding school.
- Everyone wore hacking jackets to hack in.
- None of the teenage girls seem to care much about make up or clothes.
- None of the horses or ponies seemed to be a particular breed.
- If a pony threw a shoe it was taken to the local blacksmiths.
- Everyone went everywhere in collected walk when they weren't jumping 4'6 and doing flying changes on 13hh ponies they never grew out.
- People seemed able to call the vet out without worrying about the cost or insurance premiums.
I also blame the genre for making me grow up thinking I could buy a top show jumper for £40 - bloody inflation!
I really do wish people still said things like "Gosh", "golly", and "oh rather" and had dinner at lunch time and supper at dinner time.... alas I was born a few generations too late
I still read them every now and again and I'm 26 I got the first two books as a child, then gradually acquired the rest from a secondhand book shop (where are they these days!) I remember it took me ages to get 'Jill has Two Ponies' so read them out of order wondering where on earth Rapide came from! I'm a big kid I know!To address a couple of your points
My bf still uses the term gosh - she's 30
As a child (under 10) I sued to hack out in a polo neck and hacking jacket - no doubt from Jill's influence
I'm sure Jill used to go on about being really poor, as her mum wrote books about 'ghastly' girls if I remember - can't have been too poor to afford all the above as you say, along with a house with land and stables
Although I have to say I used to get a bit upset about my progress reading about how she only started riding at 11 and by 14 could basically school a pony to advanced level dressage!
also am currently sporting a cream polo neck 
- Every adult man was a Captain or a Major and carried a shooting stick.
- People could afford a couple of lessons a week at the local riding school.
- Everyone wore hacking jackets to hack in.
- None of the teenage girls seem to care much about make up or clothes.
- In fact there was no real concept of teenagers - you were a child until you were 16 and then basically you were expected to act like an adult.
- 25 was considered pretty darn old.
- None of the horses or ponies seemed to be a particular breed.
- If a pony threw a shoe it was taken to the local blacksmiths.
- Everyone went everywhere in collected walk when they weren't jumping 4'6 and doing flying changes on 13hh ponies they never grew out.
- People seemed able to call the vet out without worrying about the cost or insurance premiums.
To address a couple of your points
My bf still uses the term gosh - she's 30
As a child (under 10) I sued to hack out in a polo neck and hacking jacket - no doubt from Jill's influence
I'm sure Jill used to go on about being really poor, as her mum wrote books about 'ghastly' girls if I remember - can't have been too poor to afford all the above as you say, along with a house with land and stables
and I used to hack out in my first hacking jacket (second hand, brown tweed, think it came from a jumble sale) much to the amusement of the older children at the stables. Thirty years on I still always ride in a hairnet and gloves. I think in Jill terms I'd probably be called "priggish" or something 
My mum read these books as a kid, then got me into them. I read them so many times I virtually know them by heart
My favourite was Jill and the Perfect Pony, where Jill had to pretend to be awful snobby Amanda, so that she could borrow her perfect pony (weirdly called Plum!!) and enter a team competition with some 'jolly sporting' kids called the Lockharts.
Does anyone remember the books about Bobby (Roberta) and her horse Shelta? I can't remember the titles but they were really good.