Car seats are only ever secured with a lap belt (or with the LATCH system, if a car has one. But securing with the lap belt is always an option, even with LATCH-capably car seats.). You may have to use a device called a lock-off on the chest strap part of the car's seat belt, but this is only needed for certain types of seat belts and only to keep the chest strap out of the way so that the lap belt can be used to secure the car seat. You may have seen the lock-off being used, which might seem to indicate that a chest strap in necessary for securing a car seat, but it's not.
I'm sorry but that is very incorrect information.
US seats can be installed using lap belt only, shoulder/lap belt or LATCH. UK seats
cannot be installed using lap belt only. They do have a LATCH equivalent, but you cannot use LATCH on the UK seats. LATCH and belt install similtanously is never allowed.
I'm not sure I understand what you define as a chest strap? Do you mean the harness?
For vehicles without automatic retrator locks, a seat belt clip must be used to secure the installation. Usually it is only older vehicles without automatic locking seatbelts.
UK car seats do not have a chestclip, whereas US seats do have chestclips that buckles the harness across the chest.