Ring or radial circuit

I would suggest wiring the bedrooms as one circuit rather than separate radials for each bedroom. If it was me personally I would wire it in ring main without installing junction boxes everywhere. I dont see why you would want to wire it as a ring main then spur off for each socket as it will cost you more in materials and the time to do it that way, while at the same time removing the ability to install more sockets in the future as you cant spur off another spur on a ring main circuit.

I personally would wire in a radial, as they are from a design perspective a better method. There is a place for ring final circuits but not typically in domestic properties, even though they are common practice. Radial circuits are typically safer and have fewer potential problems. For a circuit providing power to sockets for the bedrooms, either will suffice as the electrical demand is typically not as high as in other areas such as kitchens. Also of note, junction boxes have to be accessible and are potentially an headache further down the line. It is better to have a cable as a continuous length than use junction boxes.

Ring mains are not required these days they were used after the war when cable was in short supply. Modern day electricians instal 32A radial circuits in 4mm cable or 16/20A radials in 2.5mm splitting the house up into separate radials. If you want to do a room at a time run the radial round that room and poke the feed into the next, if you need to energise one room at a time leave the feed to the next room disconnected in the final socket. Do NOT use junction boxes they are not necessary and should be avoided wherever possible.

Yes run radials to bedrooms. You can run two bedrooms say of each radial quite happily. The second room only needs a feed from first room left under floor of second room and terminated to a junction box awaiting work in 2nd room. In all my rewire nowadays the only rings we do are for kitchen worktop areas. Rings are going to disappear as AFDD devices appear. 20 A mcb or RCBO ideally and you’ve a great set up that won’t kill your entire upstairs in the event of a fault. 2.5 mm is fine on 20 A

You can use 4mm cable for a radial circuit.

Joint boxes (unless maintenance free) are NOT a good idea and NOT allowed under floors, if you get a fault they will not be easy to get at. If you are intending to do this work yourself it must be inspected at EVERY stage by the electrician who is selected to sign it off, they must also hold Third Party certification and insurance to be able to issue you with a certificate. As the previous person said, why would you T off with junction boxes under the floor? the more joints in cables the more chance of faults. All work carried out by yourself MUST comply with BS7671:2018 I would suggest you find an electrician FIRST before carrying out any work on your own, as it could cost you more money in the long run if things have to be put right afterwards