There is no evidence that Muslim households fail to complete their Census returns (either at all or correctly). In 2001 there were just 43 successful prosecutions for failing to comply. In 2011 it was said that some 400 prosecutions were “being attempted”. I cannot find out how many were successful. In any case, not a huge number among 25 million or so households and no indication how many were Muslims.
I believe the Census details are incorrect and not only in this respect. There are huge numbers of people in the UK with no entitlement to be here. None of them will appear on the census. There are also large numbers of people (particularly men residing with “single” women) who will also not appear. There are many people who, for all sorts of reasons, do not wish to engage with the authorities and/or do not want anybody to know where they are. They either fail to complete a census return or ensure they are not included on anybody else’s. In fact, so little confidence have the authorities in the accuracy of the census that some reports suggest that the 2011 effort may be the last (although inaccuracy is not the reason given):
http://www.bbc.co.uk/news/10584385
There is no reason to believe that Muslim people are any less likely to be among these people left off the count for various reasons. It is for that reason that I believe that there are far more people in the country than is shown by the census and, by extension, far more Muslims. Whether the percentage of Muslims is higher than stated is open to argument. It depends on the religious make up of the defaulters and, of course, that is not known.