I am no expert, but I believe that the karat scale is a scale of purity with respect to gold. 22 karat i think is 22 parts gold and 2 parts another metal which is 91.67% gold. If you go here ->
www.kitco.com/market (Check the London Fix price)
you can see that gold spot price is currently at £1048.91 (as of 13:12 today) per Oz of Gold.
If you take the weight of your jewellery and multiply it by 0.9167 then times that by 1048.91 it should give you a ball park figure of what the gold is worth on the current market.
E.G. If your jewellery weights 1oz
Then: 1x0.9167 = 0.9167
0.9167x1048.91 = £961.54
If someone offers you £200, you can say this weights 1oz and gold is trading at £1049/oz....
This is only an estimate as any stones etc are adding weight. But still, it should give you an idea of the region.
I would say that you should definitely check Kitco.com for the UK Spot Price the day that you go so that you know what Gold is trading for by the ounce. As Purple Bear says, shop around. If there is a coin shop near by don't discount it either.
Best o Luck.