If you will need a mortgage to buy another property when you get back in 2-5 years time, bear in mind that the FSA is already tightening lending rules and it is unlikely that you would get another loan easily in 5 years time with no UK income to show. Your exisiting loan can probably be ported to a new property in future, avoiding the hassle of finding a new lender when you are unfamiliar with new rules.
I suggest that you rent. Make sure that your lender agrees to this first.
Another thought. If you can arrange a fixed rate loan on a buy to let mortgage for the next 5 years you will know your outgoings and should have enough cash spare to cover voids.
Ref tax, yes an agent has to deduct 20% from your rental income, but if the property has been your main residence in UK and you later let it out, you can disregard the last 3 years profit when you sell, plus £40k residency exemtion. If you own it jointly the £40k doubles as you each have the allowance. (This is how the MPs have legitimately got away with CGT on their flats etc.)
Even if the tax rules change while you are away you will still have a property in UK which is one of the most valuable things you can own. I dont know how old you are but the ability to transfer the mortgage to another property; the option to draw off more capital by remortgaging later; and the eventual option of equity release when you are much older all combine to make selling a foolish choice, particularly in today's markets.
Ask around for an agent, get references, make sure they are a member of a national professional body.
KEEP IT.

I think you should sell now keeping the present London property market condition in mind. It might drop while you are away and there could be some more issues if you rent for 5 years. Think like a professional or consult your problem with a professional tax advisor. The rules on tax change depend on UK rights, and this is in turn affected by what you continue to do in the UK when you are abroad.