After rereading your post again, it’s not clear how you are planning to offer your services to your customers. For example, are customers sending their scales to you, or are you going to do calibration on site? You’ll also have to decide which types of scales that you want to calibrate for these customers. All of them, from small gram scales to industrial food scales in the thousands of kilo range, or just a selected range? The legal requirements may differ quite a lot depending on what you choose, and they may even tell you how shipping and handling needs to be done (for the reasons that Daniel offered).
Have you considered modifying your customer scope based on which types of scales that you’re able to support right off the bat (using the smallest amount of investment possible)? Or perhaps you want to be the “expert” in a particular scale range and focus your business there instead; investing as much as it takes to become the “local expert” that your customer’s are looking for? Have you taken a look at your business risks choosing the one service from the other?
The UKAS LAB 14 document that Harry provided can be used as a guide to which ever type of service you choose.
Summary:
Find out what the minimum requirements are for the calibration services that you want to provide (contact UKAS).
Go over the spectrum of services that you are interested in and weigh the risks verses the payoffs, the investments verses the speed that you wish to roll-out your services, etc. For example, being certified to ISO-17025 may not be required by the UK for the type of customers (scales) that you’re looking to cater for. It could enough for your company to just comply to the main parts of the standard - the ones providing the best payoff.
Don’t go all the way if you don’t have to. Certification is just another business investment that should be weighed against all of your other ones and “Compliance” is just another pathway.
Think things through before starting full speed ahead. Good luck again.