Are the Newton fellowships (UK) prestigious enough to warrant low post-doc pay?
Where did you get your idea that the lower end of the post-doc pay scale starts at 28 - 29k? Most sources I've seen gives 28-29 the average pay in 2010 and 2011, and the low end near 23 - 24. (High end 35). The numbers are a bit field specific (biology tend to be around 25 - 29 for first postdocs) so I don't know whether the national average applies. 
According to the UCL webpage the Newton is tax free. Your stipend would fall into the 20% band were it taxable, so you should compare it to a roughly 30K starting salary for normal PostDocs. 
At effectively 30K, the pay is extremely competitive for early career (read first time postdoctoral position) holders, provided you don't live in London. (Oxford is not that bad.) London allowance is usually worth around 3 - 4k, so if you live in London your pay would be comparable to the lower-end of a reasonable post-doc salary (with London allowance included).