Unfortunately, with a two party system, and the candidates pretty well known years in advance (normally), I think we have to look at alternative markets in the UK, as opposed to the US where we have quite a few runners and riders, unless of course, you want to tie up money at short prices for long periods of time. Hence the question being "when will it be", not "who will win it"
The reason for seizing on this particular market is because BF have one up for peanuts and it can hardly be called a market such is the level of liquidity (well it can't) but if the same preference and prices are reflected in the high street, then I'd have thought the 10/1 about it being between July 2008 and December might be a decent bet?
The clue I believe is in Brown's last budget, where for the first time in years middle income earners in Southern constiuencies like myself actually got something
However, these tax breaks/ cuts, only come into effect in April. He knew when he presented that budget he would be the Prime Minister who'd have to win the next election, and it's inconceivable that he won't have built something into it for himself to campaign off, hence the April 2008 introduction date.
The Autumn would be just about long enough for us to notice?
Also the Autumn will dovetail with an American election that is likely to see the Democrats returned, which I can only imagine will boost Labours chances. Despite what we might like to think, we tend to ape America in a lot of areas and feel decidely uncomfortable with ourselves as a country, if we're too far out of step with them. In any case, no foreign politican(s) are more synonymous with the new labour project than the Clintons, and if she looks like winning the Whitehouse in November then I can't see it being anything other than a boost for Brown, as I'm sure we'll reckon we'll be Europes primary beneficaries etc (only British politicans and media use the phrase 'special relationship' incidentally, the Americans only use it at our request when it's convenient for us by way of sop). None the less, I think despite everything we might say and moan about them, we're actually quite precious about it, not the least because it annoys European Governments who might otherwise seek to out-flank us.
Before he can call an election though, he's going to have to have started the process of 'bringing the boys back' which means that the Spring could well be spent preparing that, especially if a Democrat administration starts doing the same about 6 months later. No politican in their right mind would want to be last man out of Iraq, as the withdrawal is likely to be even more chaotic than the invasion and occupation. So if Brown can start that process in the Spring, ahead of the Americans, it would seem shrewd to do so. Especially as he can afford to ***** Bush off electorally as he'll be a complete lame duck by then anyway.
The economic forecasts for the medium term suggest trouble, so going in 2008 rather than 2009 might have additional advantages, and now that the Cameron bounce has seemingly punctured (it remains to be seen if this is temporary or the start of a wider Tory neurosis) he doesn't look quite so formidable all of a sudden.
Recent behind the scenes appointments, and taskings to key aides at least suggest Brown's giving himself the option of calling a quick election, but having waited so long for the keys to number 10, I can't see him risking it this Autumn. If he goes soon, I suspect it will be May 2008, but by then he'll be through his own honeymoon. Provided he hasn't encountered any serious banana skins and the economy's looking healthy, I suspect he could get a mandate still though without that much uncertainty. It's just that the chances of getting a bigger one would appear greater in October for the reasons outlined?
My guess is that since the tax cuts were of his own strategic design, (in politics, egomaniacs always hold in higher regard their own ideas rather than other people's) he'll want to give them a bit of time to butter us up, which points to the Autumn of 2008