I'm not sure how much it is in England, try googling 3D/4D ultrasounds in England, or you can look in the phone book. I live in Reno, Nevada and they are $245 here for a 30min session, a dvd of the whole session, and pictures. I know that they will do a gender 3D/4D ultrasound for $145 but that's just to find out the gender, but you get photos, and a dvd of the session which is only 15min. Also they usually schedule the 3D/4D ultrasounds between 28-32 weeks, but generally like to have them done at 28 weeks so that the baby isn't all smashed, and still has room to move around. Also as soon as you find a place that does it, book an appointment, it is very hard to get an appointment when your very close to 28-32 weeks. Hope this helps!
talk to your ob/gyn they should know the prices.