As Cotedazur says, you're not going to get 6% in any short term savings at the mo, and even bonds for up to 3 years are only paying about 3%. Whilst you are right to read the small print and try to avoid early redemption charges on your mortgage, do do the maths.
Anyone who knows about finances, feel free to correct me but...
If you're allowed to overpay by 10% each year without incurring a charge, and your mortgage is £200k, then you can pay off £20k without incurring any charge.
If any early repayment over that is charged at 1%, might it still work out better to pay off more of the mortgage and take the hit on the early repayment charge if it is that low.
For example, if you invested £20k for 2 years at 3% you'd have £21218, earning £1218 gross, just under a grand if you're a basic rate tax payer (but if one of you isn't working, clearly you'd put it in their name).
However, if you're being charged 6% interest on your mortgage, then over 2 years the interest on £20k would be £2472. Therefore even after paying a 1% early payment fee (£2k), you'd have saved £472 in repayments over those 2 years, and have reduced your mortgage for the next 22 years - which for £20k would be £34,431 of interest if charged at 2.5% (which is the lowest mortgage on the market at the mo, apparently).
But if it galls you to pay those thieving bs more money for the pleasure of tying you into a mortgage at an extortionate rate, you could invest in a bond for 2 years, and then pay off another lump, and move the whole thing to another set of thieving b**s.