DNS only / No just porn!
If it's only using DNS, on the base that BT doesn't also block traffic to any non-BT DNS, just point your PC (or whatever device) DNS client to 184.108.40.206 (Google) or 220.127.116.11 (OpenDNS).
I expect the BT filter to be a bit more efficient than that and filter the IP traffic to address hosting a banned site.
I just had a look at the BT filter, it comes with 3 level. Light, med, strict. (http://www.ispreview.co.uk/wp-content/gallery/2013-article-illustrations/bt_filter_table.jpg)
Even the lightest option filter not only porn but also: Obscene and tasteless (that's great as tasteless is such an easy thing to decide.... The government insisting that the NHS and we are better of now, insisting that inequalities have been reduced since they arrived in power is obscene and tasteless to me), Hate and self-harm, Drugs (I suspect that include drug information/education/charities sites not just Silk Road), Dating.
You can add Nudity, Weapons and violence (which obviously is far less harmful/questionable than sex...), Gambling (which should be illegal to <18yo anyway....), Social Networking with the "Moderate" settings.
The strict top up the list with Fashion and beauty, File Sharing, Games and Media streaming.... (WTF?)
Sex education and Search engines blocking are optionals.....
So even the lowest setting is far more restrictive than porn.
On the other hand, a (free) AWS instance, a VPN to it; or just a Usenet client will give you access to all the porn you want.
So glad I'm an AA ISP customer....