MS has a place. Look at the requirements, and if MS is the 'best', use them. But... the 'best' definition needs to be clear at all levels, and understood by everyone involved.
My argument for migrating towards more Linux/opensourceness would be one of 'keep the money local'. Especially in the early stages of a migration, there may in fact be a short term increase in spending, to find local talent to train and reprogram systems. After the initial transition, the need for fewer license renewals can be substantial.
Assuming there wasn't 100% reduction in licensing costs. Let's say there was a 60% reduction in licensing costs, but in reality that became only 70% because of increased training, customization and programming costs. You've saved 30% but also kept more money flowing in the UK economy.
At some point, stuff need to be paid for. You can choose to hire local consultants and companies who use MS tools, meaning you're paying for local talent and paying license fees over to an US-based company. Or you can pay local talent to use free technology.
Bottom line: You have to pay talent/labor, but you may have more money in the budget to pay more local people to get more done after migrating to free technologies.