I recommend a 140cm staff if you get a 5 foot it will make behind the back passes a bit tricky as ends may hit the floor.
Suggest as you are in UK you speak to Firetoys they often make their own kit so could possibly knock one up for you although they have a good range of medium length sticks they do standard 4 foot and a big burner that is 4.5 foot (137.5cm) but big ends may make fast spinning tricky
http://www.firetoys.co.uk/juggling/aluminium_fire_staffs.htmlAlternatively get some metal pipe and wooden dowel, cut to desired length and get some kevlar (no more than 50cm per wick needed) wrap the ends drill through and fix with screws ensuring ends are flush using counter sunk washers so no sharp edges and grip with tennis racquet grip.
Ideally get a broom handle and keep cutting off short lengths until it feels good and doesn't hit the ground when you pass behind your back then measure and get a staff that length - Ta Da