Registration itself is quite cheap per sheep and can be done at any age, but membership of the Shetland Sheep Society (necessary to register) is 30 quid per year for not much else other than the pleasure of being able to register your sheep, so I would seriously think about registering them (or asking the breeder to do so for you, s/he may not be a member anymore). Unless you are seriously going into breeding AND showing your sheep, you will also be unlikely to achieve a significant difference in sale price between registered and unregistered sheep in the future.
What I am saying is - keep them unregistered (and pay for them accordingly), and enjoy them as they are... if you change your mind later re breeding and showing, you can always eat the unregistered stock (they taste the same) and buy in registered stock and become a member yourself.