There is a long history of intolerance and aggression towards itinerant people in this country. Travelling families Romanies, and cultural travellers (particularly those who stay in one place for an extended time) have variously been the subject of targeted laws and essentially get run out of nearly everywhere. Furthermore, these aren't homogeneous groups; violence between travelling families and cultural travellers is well documented and continues to this day.
I'm not saying that any of the vandwellers on here are or identify with those cultures, however it remains that the particular stigma causes a lot of trouble in the UK, and creates barriers to finding things such as overnighting spots, decent water supplies, etc.
Nearly all of the land in the UK is in private ownership, including those managed by public bodies for example the national parks or Forestry Commission land. It's not much good to say 'find a lay-by', when that lay-by may be subject to unposted restrictions or bylaws. Local knowledge is needed to determine where it is safe and permitted, or simply to give warnings (arrive late/leave early &c) about how to avoid a knock from a ranger or the police.
Most, transit facilities and other private camping facilities will not accept self builds, and have groups who are not particularly amiable to new travellers or vandwellers (be they motor caravaners, or travelling families). The only real way to find out is to speak to them. Many of us already have spoken to them, why not share that information?
Finding suitable parking in many residential areas is almost impossible in some places (permit parking is de joure in many towns due to the high population density and because most of the roads were built before the advent of the motor car). Local knowledge is very important; there is a street near me where overnighting is well tolerated but all around that area are permit only spaces.
Non-motorway truck stops, once ubiquitous, are now few and far between and no decent national list exists; many of the remaining ones don't use Google or even the Yellow Pages. Motorway services/truck stops are very expensive. The only way to find out where they are and what they provide is to actually visit them.
Public water facilities have largely been closed off. Looking in the graveyards at older churches usually proves fruitful, but this isn't universal. I personally know of four good public standpipes in my area, but visitors might not.
Public showers exist, but I only know the location of a few. More would be nice; even though I'm happy with a flannel and a bucket, sometimes it's nice to have a proper shower.
It's all well and good teaching someone how to fish, but even the best looking salmon runs can be deceptively bare.