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I think we can be sure it is Phorbia.
Looking at the shape of the phallus in combination with the shape of the V sternite and cerci/surstyly, I only can say for now...it most resemble P. nuditibia.
(which is a valid name) and till now only found in Britain.
Could I make a mistake with a similar species.....yes, because identification from photos always can be tricky!
In this case, I could be mistaken with P. moliniaris but than there should be a strong pv apical on the hind tibia (which is not present in P. nuditibia).
Just take your time with this one.....if you can you can make some more photos, but when it gets to difficult....just send it to me.

Johanna