The UK already has about 2.7 billion pounds of perpetual gilts which never mature, mostly from WW1.
Some commercial entities in the US have issued 100 year bonds, and Mexico did an issue last year IIRC.
I don't think the maturity matters as much as the amount, the OBR will publish data next week on the projected increase in debt servicing costs over the next 5 years.
Osborne seems to be using this for a bit of financial engineering to temporarily reduce the interest payments on our national debt.
This is kicking the tin can down the road (albeit for understandable reasons). The risk in all this is that some future chancellor [hi, Ed Balls - you prick] will start to use this as a mechanism to shuffle debt off into the future without addressing the root of the problem - excessive government spending.
You don't need a crystal ball to see this will start as a short term solution to current problems and end up as another bit of financial stupidity for future generations to sort out.
100 years is too long to be much use to pension funds for their Asset Liability Matching (your pension), on the other hand it would help Gilt/Swaps traders extend their curves... It would be interesting to see how a quant interpolates the 50/100 year Gilt.