Interesting that you think Blair returning will mean a troop contribution to the UN forces from the UK will become likely.
However even if he should make a contribution then it would be small and perhaps only in response to filling a "capability need" in that those managing the UN forces will ask for something that UK forces specialise in. I am no expert on the different branches of the army but i am sure we have specialism's which they will need.
Also to add, following the deployment of UK forces to help evacuate UK and commonwealth citizens in july it was announced that british forces would play no role in any peacekeeping force. The main reason for this is that britain has too many global commitments to allow any substantial force to be sent.
Furthermore one must consider the cost implications, these being more indirect rather than direct as when UK troops are on duty with the UN the Uk government is paid by the UN for the use of their forces. The in direct costs would result from the need to shape logistical systems better to fit another commitment into an already over burdened defence system.