No idea on the local companies I'm afraid, but I can tell you a bit about the cancellations etc.
They will only fly (obviously) when conditions are right, which means winds of below 12-15 knots at most, and when there is little or no chance of sudden down-draughts.
On average, a commercial balloon will only be able to fly 3-4 days a week, over a year, and will generally put you on stand-by the day or two days before a booked slot.....depending o9n company, you'll normally get a phone call the night before and then a check in call the next morning (assuming a morning flight).
Balloons only fly in early mornings or evenings, partly because of air conditions and also because the envelopes have a very limited life......500 hours or so, which at the price they are, is quite scary......The hours on an envelope are regulated and the materials are affected by UV light, so flying out of the peak sun is usual.
You usually book up at least 2-3+ weeks before a flight, and be prepared to wait several weeks after your booking date if the weather is unpredictable
The locations are not at all guaranteed.....even if a company is based in a certain place, they will have to shift their launch sites a fair bit to aim for a suitable landing ground, but in your area, you shouldn't have too much of a problem....commercial flights usually stick between 1k and 3k feet, and it all pretty much looks the same from there
If you can find a sports balloonist rather than a purely commercial operator, the chances are you'll get a better flight.....they tend to be a little easier on conditions and flexibility, although landing in a 4 man sports basket is distinctly more 'fun' than in a 12 man champagne tour basket
The biggest factor when booking a flight for someone else as a prezzy is just knowing that the chances of getting a flight on a specific date are variable.......patience and flexibility are key
PS I know some very good (Competition winning) pilots in or around Horsham and Arundel if you can persuade your mum to come to you
(I got 3/4 of the way through my FAA license before my brain blew up the first time....I started off as a chase vehicle driver in return for getting hours in my logbook in a sports basket then did the first 3 exams, then had to cut it all short 
This was 20 years ago though, so things may well have changed 
)
