Most tyre bookings take around 30-60 minutes if you're fitting two tyres and 45-90 minutes if you're having all four fitted, though this varies with how busy the garage is and whether you need balancing or valve replacement. The fitting time will be in the booking confirmation. The garage will be able to advise you on the expected fitting time when you arrive.
Next day fitting in Whitstable is usually possible, depending on what tyres are in stock and the fitting availability at your preferred local garage. Just search by reg or by tyre size, select an available garage, and review the earliest fitting time in the appointment checkout.
Different suppliers cover different areas, so some postcodes will have Mobile tyre fitting while others will not. Look up Whitstable and see what fitting options you have. If your area and the tyres you selected qualify for Mobile fitting, it will show up next to the local garage fitting options.
The fastest method is using your registration number in the search tool, which shows the tyres suited to your vehicle. Alternatively, if you are searching without the aid of the tool, you can use the tyre size from your sidewall (example; 205/55 R16) or from your vehicle handbook and check that the load and speed ratings are the same as the current specification.