I miss those lights 😢

All of those tubes where made in the same factory and production line at the factoy on Lincoln Rd Enfield

The date code on the Philips tube is indeed 8G. The month is always the alpha character with A being January and running through to M for December, so in this case G is July. The decade is obtained from whether the number is at the start or the end of the code. Every 10 years, the number switches round. It is at the beginning of the date code in the 2010s, the 1990's, and the 1970's. It is at the end in the 2000's, the 1980s and the 2020's. So, here, your tube is 8G, so it could be either July 1978, July 1998, or July 2018. You have to make a guess to which one it would be, and from the fact that they never made T12 tubes in 2018, it must be 1998 or 1978. Looking at the printing on the tube, I dont think they had the CE mark in 1978, nor that style of printing, so I would say the tube dates to July 1998. Had the date code been E1 for example, then that could have made the manufacture date May (E) 2001, as the number being at the end of the code makes the decade 80s, 00's or 20's. The round circle with the dot in was the country of manufacture symbol for the UK, used by Philips at the time. Different countries had different symbols. I have a couple of 20W Philips T12 tubes here from 1999 made in Poland, which use a square symbol with its bottom edge removed as the country code. I dont know the exact way to date the dash and dot system from the other manufacturer. The top tube is likely the newest as it shows the relatively modern RoHS and bin symbol, meaning dont dispose of in household waste as it is Restriction of Hazardous Substances - ie, it contains mercury and should be disposed of at a waste recycling centre. Colour code on the Philips and GE is 35, which is universal, or just simply standard white (3500K) and low CRI, meaning they were the old fashioned, harsh halo-phosphor type. The top one has colour code 835 so is still plain white, but the 8 denotes high CRI, tri-phosphor coating, so it would produce a much better quality light that would bring out colours in objects much better. Warm white is 3000K. Extra warm white (used for most CFL's) is 2700K or 827 for high colour rendering. Cool White I think is 5000K and Daylight is 6500K Interesting though as the power of the 8 foot tube is 100W. You would have thought as it was double the length of a 4 foot tube it would be double the power usage of a 4 foot T12 which used 40W, so it would have theoretically been 80W for double the length of a 4 foot tube, but an 8 foot uses an extra 20W on top for the same effective length. I suppose that would have been balanced out by cheaper install and relamping costs as only half the number of lamps and fittings would have been needed to cover a given area needing lighting.