Neutering sows is a far more invasive operation than neutering and therefore much less performed. Sows are usually spayed only for medical reasons. As far as I know, neutering was less risky option!
It very much depends on the vet and his experience how high the risk of post op complications is - but I would certainly not use that vet. While complications are sadly not all that rare, that ratio is very, very bad even for a general vet with not great experience! With the right vet, the risk is much lower, but like with any operation, nothing is ever completely failsafe.
Boar dating - when done properly - would be my first option and is highly successful when done properly. If I were you, I would contact Jenny (crawleygprescue) from Crawley GP Rescue near Gatwick airport; she is the rescue in the South that I can recommend most warmly!
http://crawleyguineapigrescue.org/default.aspx
It is well worth going the extra length to have things done properly - a rescue that is boar dating to the highest standard will introduce your boar to likely candidates over the course of a week, with suitable breaks in between if things don't work out with the potential. If two boars hit it off, their bond will be tested for stability, usually for a minimum of two days before they are sent home. If your boar doesn't hit it off with any of the boars he is introduced to, he will be invited to come again at a later date for a second round with different boars. So far, I haven't heard of a boar needing a third trip yet.
If things go wrong, you can always return a boar back to his rescue. When carefully done like this, the success rate is actually very high and the procedure is very low risk for you and your boar. Compared to what you would pay for an operation (and especially potential complications), paying that bit more to use a good rescue is money well spent in my opinion!