As an alternative there is a "Y Adapter" avaliable that plugs inbetween the
radio and aerial which gives the GPS unit the use of the main car antenna
(improving reception).

A car antenna is not going to improve GPS reception.

The "splitter" you are referring to allows you to connect to a car
antenna incorporating an active GPS antenna in its base. A normal
antenna is utterly useless.

Agreed if the antenna was trying to receive gps however the splitter is not
for GPS reception but FM/RDS

The unit in question is a combined GPS & TMC/RDS

The TMC system receives broadcasts from the FM frequency (classic FM)
containing traffic info.