Whether an LM subscription is bundled with the device or purchased later is totally immaterial because the conditions are identical as to the definition by Garmin for the entitled device's "lifetime". However, what i can say for sure is that many of those life-expiring conditions are not actively enforced (such as the 24 months minimum update bogy and also transfer of ownership of the device itself - they just wont let you transfer the LM between devices, naturally). What is passively enforced by simple necessity is the memory capability requirement. Should internal space limitations become an issue for later devices, their firmware is capable of the option to load detail maps directly to an inserted media card when using GE but older ones like your 300 don't have that and there's no way Garmin will ever take the time writing a few bytes of simple code to modify your device's firmware and then issue an update.

So you either ditch the bulletproof reliable nuvi 300 with it's convenient external antenna socket and replace it with a newer but crappier quality automotive device (and hope for the best for decent satellite signal reception), or bat on using the kludgy workaround i've suggested. I know what i'd do. Likely your next car after you dump that Froggy PoS will have an in-dash GPS anyway ..... . Let us know if you need any more help with the PC map data download, making a UK only map IMG file etc.. Cheers Mate.