Evaluation criteria for location of supplier (distance from requirements)

Hi all, looking for some guidance if possible. I have an IT requirement that requires the supplier to be within a certain distance or travel time of the service delivery address (network/data centre in close proximity to customer). My question is how can you define this criteria fairly within a tender document i.e. ITT? For example, can I request that the supplier be within 50 miles of the delivery location or is this restricting competition? The reason I would like to know this is because the requirement is based in London and a supplier based in Scotland can deliver the service no problem, however it would take one of our engineers far too long to get to Scotland if a problem occurs. Therefore distance to the delivery location is vital and needs to be scored against. Requirement is in OJEU thresholds, so I need to gain some confidence in this issue. Any guidance or advice is very welcome