There are very few absolute, categorical musts in public policy. Although politicians and commentators prefer such statements - one example is the ludicrous and repeated statement that we just 'have' to recycle more - real life almost invariably requires balancing costs and benefits.
I'm pretty certain that what Brown really meant was that there is no alternative to nuclear power if you want to obtain serious cuts in carbon emissions without the economic costs being too high. This is a very plausible position and one that Tatchell's article does little to address. After all, it doesn't contain any numbers. None of his different measures comes with a price tag or a carbon emission saving attached.
The big difference between nuclear power and most renewable options is well summed up by this graph, via Wat Tyler:
That is why anyone serious about cutting carbon emissions - including, to his credit, the rather messianic originator of the Gaia theory James Lovelock - should think very seriously about nuclear power. It is, by a significant margin, the most economic low-carbon method for producing energy.