There are plenty of counterfeits out there of the 1887 Five Pound. If the bead at the very top of the crown touches the denticle above it, then it's a fake. There are probably other examples, but that's one I have seen firsthand.
How do you know? In other denominations there are die varieties where the crown can touch or not touch the rim.
Local coin dealer had shown me one many years ago. Color didn't look right either. I can't imagine there were very many dies used, the mintage was considerably less than 100k.
FWIW, here is a scan and photo of a 'Beirut' copy of the £5 and a document produced showing the results of an analysis by the Royal Mint. As you can see, the weight is slightly down, the milling count was 188 instead of 184 and the die axis is off. FEATURE The counterfeiting of British Victorian £5 gold coins in the 1960's The Public Records Office, Kew, has been recently given a "make over" and renamed the National Archives. Here are deposited the record books and official files of the Royal Mint. British law usually allows these to be examined after thirty years. One of these files, prosaically named: "Requests for examination of £5 pieces", Ref.2, allows us to find out about the glut of counterfeit £5 pieces which entered Britain in the late 1960's. The file is documented to cover the periods 1965 to 1969. It starts with a request, in November 1965, from the Customs and Excise to the Royal Mint to examine a 1887 £5 gold piece. This piece was one of a number imported from Kuwait by a Mrs.Akel, a Birmingham jeweller. It was alleged she was selling these pieces on to other small jewellers in the English Midlands. G.P. Warden, a principal scientific officer at the Mint, reported the piece was a counterfeit. This was based on the low weight and density of the piece, the incorrect number of millings on the edge and a number of visual defects on the coin. From the density of 17.05g/cc Mr.Warden estimated that the coin contained about 89% gold as against the 91.66% found in the genuine coins. The file contained a photograph of this coin and it is reproduced below. Photograph showing the 1887 Jubilee gold £5 ex. Mrs.Akel Type Mrs.Akel counterfeit Genuine coin Weight 39.7204g 39.87549 to 40.00507g Density 17.05g/cc 17.45 to 17.55g/cc Millings 188 184 The file does not detail the visual faults of the counterfeit but examination of the photograph reveals a number. On the reverse, the body of St.George had not been completely "made" during the striking operation. Both sides contained a large number of pimples and depressions. The pimples were especially noticable on the table next to the body and leg of St.George and on the bottom part of Queen Victoria's veil. There is also a small die crack visible near the top right hand side of the I of Victoria.
Hm, it might have been another year but someone posted some stats from the Royal Mint for five pound production one year and there was an outrageous number of dies used - I think they struck less than 200 coins per die pair on average.