P2P Technologie, recap ?

Your first port of call should really be speaking to your compliance officer / advisor.  The FCA have some very strict regulations around certain aspects of the system, such as their client money rules, which you should be designing your system around.  You need to make sure you understand SQL databases, particularly ACID compliance, foreign keys, indexing and transactions.  Naturally there are other approaches but given the nature of the system you're looking at tackling, and your (assumed) relative inexperience, you need to be relying upon your database for your information integrity instead of trying to invent those systems yourself.

In terms of the language or framework you should use - they're all pretty good these days and all are quite capable of building a P2P website.  They'll have their own strengths and weaknesses but the most important factors will be what you're comfortable using and how easy it will be to recruit other developers and bring them up to speed.

As web projects go there aren't too many that are more difficult or time consuming than building a fully compliant fully featured P2P platform with all the associated administration and CRM facilities required by the business behind the scenes.  I've seen several experienced agencies completely underestimate the work involved.  There are off the shelf solutions available, although I've not used or explored any of them personally, and as far as I'm aware they're not used by any businesses that have been through the FCAs full authorisation process.  I'm sure a quick Google search would track them down.  If you explore that route it may be better to wait a few months until they've shown themselves to be fully compliant to avoid interruption down the road either through being shut down (however unlikely) or through having to make major changes to your platform and processes.