I think Stansted's layout doesn't make it very easy to provide an AVP. Regarding the Trump visit and Obama before him, Stansted was seen as the best London airport alternative to Heathrow where the disruption caused by previous US presidential visits was significant and not acceptable. Also Mildenhall and Lakenheath military bases being nearby to Stansted provide convenient facilities for supporting US presidential aircraft. For info Manchester, as well as a few other UK airports, is also a "nominated" diversion airport in the event of an aircraft experiencing a security issue.