This is a really interesting thread - although obv on a horrible subject.
Not sure how much I want to get into it, because I'm not up to date on all the latest statistics and articles - if people ask me questions then I might not be able to come back on them. But I would like to say something about the idea we've discussed of whether Britian is 'full' or close to being so. I would never want to claim that describing Britian as full, swamped, crowded etc is in itself racist, and to do so doesn't help the debate at all. However, whether intentionally or not, imo these terms do unfortunately tend to monger fear, and feed into racism in the way that they may lead people to view immigrants as a thread, something to be wary of.
I reckon there are two main problems with the claim that immigration is filling up Britian. The first is that it treats Britian as a black box in terms of population. This is misleading because my understanding is that if there is a population problem in the UK, it is not one of population level or population density, but population distribution: the fact that so much of the population is crowded into a few south-central English cities.
(Of course, there are various reasons for this, and not all of them are easily overcome. Tea mentioned that a lot of the underpopulated land is used for agriculture - however, I don't agree that all of Britian's agriculture is hugely intensive, in the case of sheep and cattle farming areas especially there is still some ammount of 'free space' that could be utilised without preventing farming taking place. Other reasons for not building could include poor economic prospects, the environmental impact of rural expansion, and the fact that some of the wilder/more remote rural areas are just not very hospitable environments for large-scale settlement. However, the fact remains that there is land there if required.
Also, it's not just a case of urban versus rural. Many urban cities have under-utilised capacities. It's widely know that the population here in Glasgow has been steadily declining for a long time, and there's consequently large ammounts of both vacant housing and abandoned brownfield land that could be made use of. Again, there are obvious reasons why some cities are less populous than others, to do with trends of industrial/economic decline and growth. Some of the inequalities in these factors are prob impossible to solve, some are but would require strong government intervention of a kind that some parts of the political spectrum might not approve of. But again, the point is that it's not a wholly insurmountable problem).
The second problem is that the claim seems to assume that immigration will continue in the future at either the present rate or higher, when in fact there is no reason to assume that this will be the case. As m_b pointed out, people will on migrate en masse based on perceived relative advantages of the destination, and these will tend to relate to the quality of life there. If indeed the ammount of surplus level of jobs and housing is decreasing, then that in turn gives less reason for people to continue to move here, leading to decreased immigration rates in the future. Without wanting to be flippant, it's entirely possible that the problem could 'solve itself' in the near future, albeit perhaps not in the most pleasent way.
There was another thread, I forget which, where the recent Polish migrations (which we are all told were going to 'swamp' us, of course) were being discussed, and someone pointed out that a large number of Polish immigrants have already returned home, having found the economic conditions here not nearly as positive as they'd hoped/expected. (A further factor, of course, that affects which countries are most attractive to migrants is how easy or otherwise it is to access them. However in this regard, it ought to be pointed out that the British immigration system, while rightly more just than some, is far from being the 'soft touch' that its right-wing critics claim).
A further point that ought to be considered is that an increase in the rate of migration does not by itself mean that the entire population level of the country will rise. It needs to be weighed against other population trends, and has been pointed out many times, the birth rate amongst indigenous UK citizens has massively declined in recent decades.
One more thing I'd like to mention, more directly connected to the BNP - it is often assumed that the BNP's core support comes from a disenfranchised white working class, looking for radical answers to long-term problems with unemployment and poor housing provision. This perception, I would argue, has lead some liberal critics to hold back from fully taking on the BNP, both out of a general feeling of guilt or sympathy towards its supporters, and a fear of further alienating them.
But in fact, whilst yet again I don't have the figures to hand, I've heard of several studies that suggest a large percentage of the BNP's support comes from people of more lower middle-class backgrounds. In this case, the presumable reasons for support would be far less creditable, ranging from straightforward racism, mongered fears about Islamification and political correctness, and perhaps that feeling of a 'loss of traditional priviledge' that was discussed in the American health care thread - the world is changing, the country doesn't solely belong to 'us' anymore, and we're scared.