To answer Pembroke's question: This looks like an error to me.
No airway should be defined with a FL base below the TA. This airway section is within the London TMA (TA 6000ft), so a FL55 base is wrong. Also, a TMA and a CTA should encompass all airways within there lateral boundaries below their top level, although I am not sure that this is a hard and fast rule.
I have an idea how that might have happened: That part of the airway is NOW within the London TMA, but until recently it was part of the Worthing CTA, where the TA is 3000ft. That part of the CTA was redesignated to London TMA in 2012, I believe, but someone forgot to change the base levels of the airways within that chunk. Evidence? the AIP entry for airway N57 at SANDY actually says
Originally Posted by UK AIP ENR 3.1-54
The base level change is ... contiguous with the base level change btn the northern and southern portions of the Worthing CTA
Except that this is now the level change between Worthing CTA and London TMA, where the TMA base is 5,500ft.
Hence the VFR chart and the IFR charts disagree, the VFR chart only shows the LTMA, the IFR chart shows the airway base, both using different sections of the AIP as equally authoritative source...
Might be worth mentioning to the CAA...