You can take out contents and buildings policies separately if you wish, simply select the cover required when obtaining a quote. Please see our other question
It makes sense to have both buildings and contents cover with the same insurer because if you need to make a claim you will usually only have to pay one excess. If your cover is split then you will have to pay an excess to each insurer.
If you need to make a claim which damages your home and contents it is a lot easier to deal with one company rather than two.