Travel system parts - don't all fit in boot

[KatieBurningham]

I can only fit the frame in the boot of the car, I can't fit the pushchair part in.

Can I secure the pushchair part on the back seat with seat belt? Whilst baby is in car seat in the back?
I'm assuming all will be ok as long as I secure it and not leave it loose but just wondered if this is a big no and instead need to find a smaller travel system?

I have a similar problem, the frame will fit in the boot but the carrycot won't - I'll probably put the carrycot into the passenger seat and the dog will have to get used to riding in the boot with the pram frame. Everybody so far has told me you shouldn't have anything in the back of the car with baby in case of a sudden stop or accident.