I suggest 'Buy British' when you can is a good policy providing manufacturers are capable of providing a product in a reasonable time and cost.
However, the UK (or should I say BAe Systems) has proved on many occasions that we are just not capable of doing either. Nimrod AEW3 and MRA4 are 2 perfect examples. If the UK decides to procure a new Maritime Patrol Aircraft in the future, there really is no other realistic option other than buying from overseas. Similarly, C-17, Reaper, Rivet Joint etc..
Bottom line is that there needs to be a balance if you want the capability.