NO we certainly do not need any more cons.
This is a list of every convention I know of in the UK and Ireland, and aside from those, there are other sci-fi cons which can overlap, as well as various small local events dotted about (such as anime club mini-cons). As you can see there are already *a lot* of conventions all over the country.
Many of these events only have say 300 attendees. The problem seems that everyone wants a convention on their doorstep. However, when there are that many events around, getting the attendees can be difficult, particularly for small, first-time cons. Small, new cons rarely get more than half of their capacity. Also they’re competing with bigger and more established conventions (Ame, Minami & Aya)
A piece of advice for your friends in Manchester, in addition to what I have already said, (from someone who has been through it before), organising a con is HARD. That is all.
Regarding what you say about other countries, remember how much larger the US is, and that Japan is a completely different situation since anime is mainstream there. These countries are simply not comparable to the UK. If you look at the rest of Europe, certainly countries such as France (where anime is a bit more established) have larger events, but not necessarily *more* of them. Much like how we have events centred on London, several of theirs are located around Paris. The number and location of cons isn’t the point, it’s their quality and therefore how many people will want to attend them.