We don't have many reindeer in Sussex, although I think you'd find some at visitor attractions like Drusilla's. We do, however, have plenty of other deer and even a few muntjac if you look carefully.
In a magazine this month, the naturalist Simon King mentions Knepp Castle in West Sussex as one of the key places in Britain to see herds of wild deer. They certainly do range around Knepp's beautiful and extensive grounds, but I would rate the herd at Petworth Park too - not so wild as the Knepp herd, but still numerous and spectacular, and easy to spot.
Equally, however, the forests of Sussex are home to vast numbers of roe deer. I always chuckle at a booklet I have about St Leonard's Forest, written in the early 1980s, that says there are perhaps 30 roe deer living there. Well, now there are a lot more than that. You might see them if you're walking quietly in the forest, as they hide and raise their fawns in among the trees and bracken; if you have dogs with you, please keep them under control as they love to flush out deer and other wildlife. Also if you're mountain biking in Sussex please be responsible - it's fun to follow the small tracks but do remember they are deer tracks, and deer are very shy.
This is why the place you'll definitely see them is around the forest edges. More activity on the forest itself in the past few years, from recreation to forestry work, has driven them down into the surrounding fields which now resemble a deer park. Many of them are white, too.
From some footpaths you can see them grazing in fields alongside horses. Deer eat rather as sheep do, nibbling new shoots of grass when it's available but preferring leaves, twigs and broad-leaf weeds, whereas horses are quite fussy; they say sheep are a good grazing companion for horses for this reason, so perhaps deer fulfil the same function. A nice symbiotic relationship!