Wiltshire is chalk downland country with a strong Pony Club culture and a genuinely central position, making it easy to reach from the south-west, the south-east and the Midlands alike. One quirk worth knowing: Salisbury Plain is a large working military training area, and access for riding is restricted and sometimes closed — if a pony has been kept near the Plain, ask specifically about hacking access rather than assuming open countryside.
Away from the Plain, Wiltshire offers good, honest downland grazing and a solid, unshowy family-pony culture rather than an especially expensive or glamorous one. Its central location is its biggest practical advantage — a Wiltshire viewing is rarely a huge detour for buyers coming from several different directions, which is worth remembering if your own county feels short of choice.