External Flood Light Junction Box

I want to fit an external flood light and was thinking about putting junction box outside on the wall to join the cable from the light to the cable from the lighting circuit.

If the junction box has "nipples" do I need to use 20mm glands?

Should I take the twin and earth cable from the circuit into the back of the box or into the bottom?

I've read it is better to have both the cable from the light and the circuit into the bottom of the box so water cannot ingress.

If the cable goes into the bottom, as I understand it T+E is not rated for external use. Do I need an external grade cable to then connect internally onto the lighting circuit T+E?

This is a separate lighting circuit for exterior lights so there is discretion if a fault develops.

Thanks in advance.

Should I take the twin and earth cable from the circuit into the back of the box or into the bottom?
The back - see below

I've read it is better to have both the cable from the light and the circuit into the bottom of the box so water cannot ingress.
Proper glands should provide the IP rating, but the bottom is always better than the top or the side.

If the cable goes into the bottom, as I understand it T+E is not rated for external use. Do I need an external grade cable to then connect internally onto the lighting circuit T+E?
Yes, T&E is not rated for outside. Take the T&E into the back of the box (then it isnt external). The cable from the lamp cab go in to the bottom, it will have cable suitable for external use.
Drill a hole in the bottom of your box to allow any moisture etc to drain out.

Thanks for the fast reply.

Should I seal the entry in the back with some silicone ?

You can seal the entry in the back with some waterproof compound - others prefer silicone and the like.