Dark Blue and Yellow colours may be rather "yucky" but they are certainly good for conspicuity at low level. The vast majority of other Police operators and the Defence Helicopter School at Shawbury seem to agree.
I can't think of a worse colour than silver for conspicuity, it blends in beautifully with the grey British weather! I wonder what the Heathrow, Northwood and London City ATC Tower controllers are going to think? Trying to spot them visually when they are operating close in to their airfields is going to be very tricky.
There may be a fair bit of controlled airspace in London, but there is a lot of free airspace as well. Fairoaks, Biggin Hill, Elstree and Stapleford are all very busy and there is a lot of VFR traffic that routes around the edges of the Heathrow and City zones.