In my opinion, the supporters' group could encounter potential issues for using a magpie logo on the shirt and calling it "The Magpie Shirt". Newcastle United Football Club (NUFC) have trade mark registrations for "THE MAGPIES" (UK registration no. 1528761) for goods including shirts and T-shirts and a magpie logo (UK registration no. 2570729) in class 25 for clothing. The use by the supporters group of identical or very similar signs in relation to identical goods could be an infringement, even though it relates to the sale of unofficial goods or use as a badge of allegiance to the club. In Arsenal Football Club Plc v. Matthew Reed  EWCA Civ 96 a street seller was prevented from selling Arsenal memorabilia , even though he prominently displayed a sign stating that the goods were not official Arsenal goods. The crest containing three castles is less of a concern: it is part of the Coat of Arms of the City of Newcastle so alludes to the town but the NUFC emblem generally replaces this with a black and white shield.
There could also be passing off issue with the supporters group promoting the shirt as the "Magpie Shirt" with a magpie logo. NUFC are well known as "The Magpies", based on the black and white of their home strip, and may well have a reputation or goodwill in the phrase and image in relation to Newcastle United, football and football shirts. The use by the supporters group of this image and phrase in relation to a black and white football shirt might be seen as a misrepresentation that NUFC are associated with or have endorsed the shirts in some way. Football fans, who are potential buyers of the Magpie Shirts, are well-aware that clubs authorise replica shirts so calling a black and white shirt "the Magpie Shirt" could lead to the false impression that the club has endorsed it. On the other hand, the website states that it is an alternative to the official replica kit and uses the phrase "Be alternative, Be United", which might go some way towards negating any misrepresentation. Similarly, the fact that these shirts don't incorporate the logo of the official sponsor WONGA.com might lead savvy football fans to conclude this has nothing to do with the club.
Wonga itself will not be too pleased that football shirts are being sold to NUFC fans that don't incorporate its branding . However, they are unlikely to have an IP claim because there is no use of its marks or logo. Perhaps, they will be putting commercial pressure on NUFC to take action to safeguard a proper return on its sponsorship arrangements. It is a difficult issue for NUFC and the prospect of having to take legal action against its own supporters to appease sponsors and its owners, will not be an easy decision to make.
Alternative football memorabilia is not a new concept. The Old Trafford faithful displayed their protest against the Glazer's financial management of Manchester United (MUFC) back in 2010 by donning green-and-gold scarves, the original colours of Newton Heath football club. However, there weren't any trade mark or false endorsement issues as green and gold is rarely used in MUFC marketing (the 1992-94 third shirt being an exception) and the scarves didn't incorporate any other MUFC insignia.