It seems the march organisers and some of the people going on the march have similar concers as you. to quote from the site:

"Some people have expressed concern about far right groups infiltrating our rally to further their own agendas.
- First and foremost, this is a peaceful, non-political rally that will be supported by families, children, pensioners and the infirm. Although we would obviously like to see as many people as possible there, it must not be at the expense of the
integrity of the majority."

re: http://www.justiceforengland.com/may_march.htm