Firstly, owning a marmoset in Britain is legal - dependant on species whether you need a DWA license.
The moral and ethical feeling sbehind captive primate keeping are a totally different matter.
Marmosets, in the wild have a very complex social infrastructure and have extremely complicated psychological and require a great deal of care to ensure they get everything they need.
Fristly, keeping a lone marmoset is a big no-no - only a very small number of primates will successfully live alone. At best, marmosets should be kept in pairs or trios.
You would require a large outdoor enclosure for them which is secured from them escaping and from anyone getting into it to steal them.
You will need a very good vet on hand who is both experienced with marmosets and willing to treat them should an emergency arise.
Enclosure wise - you are going to be needing a large area, with plenty of environmental factors from their natural habitiat taken into account.
You will need to follow a complex diet for them and this alone with supplements and such can be expensive.
In all honesty, many people are against private primate keeping in the UK - some of the biggest primate rescue organisations and zoos are at the forefrontof leading campaigns to ban private keepers from housing primates so it isn't a job that can be taken lightly.
I would seriously consider a different species who can offer you the companionship you are seeking from an animal but who can also have it's own needs attained without such a difficult aura surrounding it.
Sugar gliders are very popular pets in the UK and are very interesting to watch and can create a strong bond with their keeper given time and patience. SKunks are also amazing animals to work and live with - we share our home with a little female who offers us all the love and affection anyone would wish for from an animal.
PLEASE consider everything carefully and should you decide you can offer a primate the space, energy, time and financial standing to care for it properly, be sure to go through a reputable breeder/broker to make sure that they are aware of the needs of the p[rimate and not the needs of thier wallet.