If you are temporarily taking goods (such as tools) out of the UK with a value over £1500, then you need to have an ATA Carnet. Below this value, you can just make an oral declaration.
Sometimes referred to as a 'passport for goods', an ATA Carnet is an international document that allows the temporary export and import of non-perishable goods for up to one year. This includes things such as tools of the trade, exhibition equipment and commercial samples.
If you are taking consumables like nails, screws and welding rods you need clearance but can claim Returned Goods Relief (RGR) on goods coming back.