Identifying eucalyptus is difficult. On the one hand, they are all a bit the same, and other other hand individual species are very variable. Gunnii itself is very variable. Then there is the problem that many of them have different shape leaves at different stages of their life, and may revert to juvenile leaves if you cut them back hard. At the end of the day, you often need to see flowers and seed capsules to be sure, and yours hasn't flowered.
Here's a good place to have a look through hardy eucalyptus types.
http://www.angelfire.com/bc/eucalyptus/
But unfortunately there are a lot of them, though only a few of them are commonly grown in Britain, especially gunnii, globulus, archeri, perriniana, nipophila, pauciflora, to a lesser degree debeuzevillei, coccifera, gregsoniana, rodwayi, delegatensis, dalrympleana, crenulata, subcrenulata, etc, etc. Though the three I have in my garden are none of those... Frankly if it is anything like one of the first few in that list, it probably is.