I'm sure there is no reason why you can't use this towards your evidence, but I wouldn't rely solely on it. Although they give regions/counties for you to choose where your house is, it doesn't take into account that there are affluent, poor and average areas in towns and counties where prices given by Halifax and Nationwide can't really be relied upon.(posted 8258 days ago)I visited Halifax and Nationwide websites, and entered my details - Halifax's estimate increased the value by 5k but Nationwide's increased it by 10k. However, the valuation done by an estate agent last month increased the value by 35k!! This last valuation can be backed up by the fact that a two houses in my street have sold for the same or slightly more in last couple of weeks.
If you can get a retro valuation then do so, but its certainly worth asking Halifax about the difference from their website valuation, especially as they claim that its relied upon by Government as well!!