Unfortunately not all car seats are compatible with all cars. You can't use a seatbelt extension to fit a car seat, that wouldn't be safe. If you've got a short seatbelt you might need to get a seat which can be fitted specifically with a short seat belt. Does your car have isofix, even though the seat you've got isn't?
I agree Car Seat Advice UK will know. You could also try going to a baby shop like Smyths, Mothercare or John Lewis and ask if you can try different seats out in your car. (I don't recommend Halfords, their fitting advice is not very good).
In the meantime have a look at the mybabiie fitting video:
And this general video for tips on installing infant carriers. Although it's not shown on the mybabiie video, the step of pushing the seat in and pulling the belt tight on the lap belt step is essential if you're looking to get a good, solid fit with a belt fitted seat.
Sometimes with infant carrier seats, you can add a step in the middle - so you hook the lap belt across, plug in,and then tip the whole seat up and forwards, towards the back of the car. The handle will usually need to be placed backwards for this step. Then you can route the seatbelt loosely through the clip on the back of the seat. Push the child seat back down into the usual position making sure the base is at the correct angle on the car seat, tighten lap belt, and smooth diagonal belt through ensuring no slack. Then make sure the carry handle is in the correct position (which is important).