Taking £700 UK retail as gospel, I see the list as...
Canon 100/2.8 (non-IS/L)
Sigma 70/2.8 ArtTamron 90/2.8 VCTamron 90/2.8 non-VCSigma 105/2.8 OSIf you can blow the budget a bit, Cotswold Cameras have the Canon 100/2.8L IS at £790, rather than the normal £910-£950.
Having said that, you'd be best off reserving some budget for tripod, focusing slide, ring(?) flash, extension tubes to go past 1:1, and other such accessories, so maybe not.
It depends to some extent what you want it for - are you thinking tripod mounted with flash setup etc, or stalking insects on flowers, or maybe general photography too?
Image stabilisation would be important for stalking - you often end up taking the shot when in an awkward position. On the other hand it's of no interest on a tripod.
The Sigma 70/2.8 Art is probably best optically (other than the over budget Canon100/2.8L), having said which, most mid-range macros are pretty good.
The Tamron 90/2.8 VC is probably the best of the rest - it's got stabilisation, some weather resistance, and I remember good reports of the optics.
I've got the cheaper Canon (second hand for less). Buying new now, on your budget, I think I'd get the Tamron 90/2.8 VC.
NB - I think Cotswold Cameras are a grey importer, if you care.
This generally means that you save a good part of what would normally be the importer's (i.e. Canon UK's) slice of the profit, and that the warranty is insurance based rather than through Canon, which probably means a harder line on accidental damage or post-expiry claims.