My opinion is Ebay, rather than Autotrader.
Put it on an auction with a very low start price and a sensible reserve, rather than a fixed price.
The less 'common o' garden ' cars (especially classic cars, and now rarer higher spec Omegas such as Elites) all seem to sell well on Ebay if advertised well.
I have sold a 51 3.2 Elite saloon, and an 03 2.6 Elite saloon, both approx. 105k, both very clean, and very pleased with the final figures (both more than reserve), and both in excess of your figure. Neither were silver though - one Nocturno Blue and the other Sapphire Black, which probably helped..... Just have to dodge the Ebay messers though.
The last one had nearly 2000 views, and around 175 watchers, so the interest is still out there for the right cars.
But as with anything that is being sold, superb photos, and a good clear and informative description are key.
Just my opinion, though - others on here may disagree.