Credit ratings for government debt don't make much difference to influencing investors. Really they just make headlines for newspapers and allow politicians to make political capital out of each other.
The reason for credit ratings is to solve a problem of information, where potential investors don't have access to enough information about the credit worthiness of a business. With government debt though there isn't an information problem as investors have access to loads of information about the relative credit worthiness of a country, because of all the economic indicators that are published. Most people would know that lending to Switzerland, Germany, Sweden etc is safer than lending to Greece, Italy, Portugal but if you were comparing between lending to private businesses, apart from the big names it would be much harder to differentiate without having the credit rating as a guide.
So whilst credit ratings can drive investor behaviour a lot with lending to private firms, we are unlikely to see much effect if any on the demand for lending to the UK. Nobody is going to think the UK is more likely to default tomorrow than today just because of this rating downgrade.
The biggest issue with this downgrade is political not economic. Osborne made himself a hostage to fortune, by staking his economic credibility on retaining the UK's AAA rating. The UK has never been downgraded before, so Osborne probably guessed he was on safe ground by making the AAA rating a benchmark of his own performance. Every time he is asked about poor growth figures, poor employment etc he always uses as his main achievement "we have retained the AAA rating due which is a signal of market confidence in our economic strategy". No Chancellor before has ever banged on about the credit rating.
Now this has come to bite him on the ass because we have lost it and so obviously he has now failed on his main claim to credibility.