Russell, Sounds to me like your lathe is the same as mine, imperial gearbox, 8tpi imperial lead screw, this makes it an imperial lathe! Of course you will need the standard imperial end gears in order to get the correct screw feeds.
Of course with the use of different end gears, as Mick said, you can cut metric threads but that doesn't make it a metric lathe. The thing that would make it metric or imperial by definition would be the Boxford name plate, if it is a metric lathe it will have a metric name plate, if it is an imperial lathe it will have an imperial name plate. Also the metric gearbox is kind of a mirror image of it's imperial counterpart, with the tumblers on opposite sides. By all accounts even some of the lathes sold and labelled as metric, with metric name plates and gearboxes, were fitted with imperial feed screws, this was done to use up the stock of feed screws already on the shelf and again they were fitted with different end gears to enable metric thread cutting.
I think I already explained this bit but I'll repeat it in greater detail. As we all know, Boxford lathes have always been used in schools,colleges and with the advent of metrication they couldn't afford to scrap all the imperial parts they already had. Also schools that already had imperial lathes couldn't afford to replace them with new metric ones or fully upgrade them by changing lead screws, gearboxes etc. The simplest thing to do was to change the dials on the cross slides and compound slides. It was a stroke of genius how they accomplished this and a case of using very simple mathematics.
On an imperial Boxford as we all know, one turn of the dial moves the tool 100 thou, or a tenth of an inch, so the dial simply has 100 increments of 1 thousandth of an inch. I'm sure we are all aware that there are 25.4 mm in an inch (approximately), therefore a tenth of an inch, or one turn of the dial, is 2.54 mm, they simply discounted the 4 hundredths of a mm per turn and rounded it down to 2.5mm, after all they were meant to be used by students not in industry. Of course they couldn't put 250 increments on the dial as they would be too close together and hard to read, so they simply made 125 increments, each increment 2 hundredths of a mm. There was no need to change any feed screws or anything else, as this was unnecessary and would have been prohibitively expensive. As Mick said all they did was add a 127 change gear and with the aid of a conversion chart (which is widely available on the internet and in the more recent versions of Know your lathe) and hey presto you could cut metric threads on your imperial lathe.
Hope this isn't too basic an explaination but it really is that simple. Rest assured, as soon as I get a tool holder adapted to hold a dial gauge I will test the theory out with hard facts but I've tried it with a magnetic mounted dial gauge and I'm getting 2.52 mm per turn. I know this isn't a true reading as the magnetic base isn't perfect but it is pointing towards a tenth of an inch per turn!!