Morris Minor Owners Club

Very interesting but "low cost contract sim?" Pay as you go with £5 or £10 should cover plenty of text messages.
A new wire to the SOS button is a good plan, especially if it saves you having to send a text every time you want to use the car.
While you are adding that wire why not break out the aerial so it can be put anywhere?

The GPS antenna in the box is probably passive patch design whcih will be mounted on PCB, difficult to remove and will not work on a long cable.

GPS antenna on a long lead are active antenna design (in built amplitier) quite a different beast from above.

the more rugged looking unit on ebay with seperate antennas does not seem to sell for much more and gives you choice over mounting and access to that SOS feature.

Looking through the feedback for the sellers there are some complaints about networks never connecting and he often replies that is must be GSM only, so you might have to choose your sim carefully