The UK needs to adopt a more federalist style regime, especially if the the incumbent administration is predicted to be replaced later this year.
Providing Scotland with both the 'benefit' of electing its own adminstations with the safeguard of the Westminster Government, especially during chaotic periods, will do more to enhance unionism than any other measure, in my opinion.
Providing more fiscal and policy related power to Scotland in terms of its own domestic matters will unlikely impact the rest of the UK and if Scotland decided to embark on an agenda which 'harmed' both relations with its nearby neighbor and implemented an uncompetitive regime, in the medium term, the UK would benefit from the fact a number of parties, including Labour, the Conservatives and the Liberal Democrats in the Westminster Parliament didn't believe independence was the answer.
Therefore, let SNP do what the SNP intends to do, give them more control and lets compare what the outcome is.
If in 5-10 years or so, due to terrible prospects for Westminster Scotland decided to leave the UK, so be it. If on the other hand Scotland pursued a combination a economic/domestic policies which damaged its reputation, then the UK could enhance its position within such a situation.
Either way, if Scotland succeeds as an independent nation or if it performs poorly, the rest of the UK, I believe, would perform well.