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This appears to be a member of Therevidae. The supra-alar processes are elongate, whereas they are truncate and resemble a short carina in Asilidae. Furthermore, note the laterally-protruding antennal sheaths (these are more posteriorly directed in Asilidae) and the slender posterior processes, which are thickened and thornlike in Asilidae.

Stubbs & Drake 2001 have a good key to families of British pupae, and in this case its diagnosis would have been correct (it doesn't always work in other parts of the world).

I wouldn't be surprised if somebody more familiar with Therevidae could identify this further.