the short answer to your question is yes, you need an IR to fly on an airway in the UK. The slightly longer answer is that the advantages of so doing are many-fold, separation, lost comms procedures, terrain clearance and radio aid reception are assured, known traffic environment, leading to instrument approaches. etc etc. The USA is different in that one can fly on airways under VFR as they are class E not A, at least in the lower levels.
Do you have/are you considering an IMC / IR(R) ? that would be the starting point to figure it all out properly. Also - if you want to see the airways and how they are defined, go to skyvector.com and click the world lo button to get the IFR chart up.