Baboon, while you'll get a lot of great info here on PPrune, I suggest you give the CAA a call re the various legal aspects, of which I'm sure there are quite a few
On a different note, I'm not sure how far you'll get with a 150/152. Depends of course on the banner size, but
AFAIK a 172/182 is more appropriate. While never banner towed myself, I've flown a/c that were used for this and IIRC the only mods were a tailhook, a release handle in the cockpit and little mirrors mounted on the struts (of a 172 in this case). HTH.