Your Ateca was built at the Skoda factory in the Czech Republic, so will be at the German port of Emden. It can be there for up to 3 weeks before being assigned to a ship. The destination port for Manchester is likely to be Grimsby. Once there it can take up to 10 days to get to your dealership. You ordered in March, so 3 to 4 months is about right in the currant climate.
To get better information on your Ateca contact Seat customer services, give them the 8 digit order number and they will tell exactly where your car is.
Hope this helps.