Actually JPS is right. Here in Sweden, SFRO (equivalent of your IVA in the UK) even recommends to turn down the column to get a perfectly smooth and regular surface before welding.
Now, if you don't have a lathe, you have to compare if it would be cheaper to buy 1m of 18mm ID thick-wall tube (and 3mm wall being the minimum, you will have a 24mm OD tube. Check in that case that you will have enough clearance beside the U2 upright tube) or to give the 2 parts of the column to an engineering shop to turn them down.
Good luck
