Realistic trade-in? Glasses Guide anyone?

When changing cars I used to use the Vauxhall website which had a link where you could get up to three valuations directly from Glasses guide. Now the website gives an average range instead which i am happy with as long as it is realistic. However, i did get a trade-in value recently which was at most about £200 less than the vauxhall price. I was told for the car i could get up to £5k but they would be more angling towards £4,500. Vauxhall web has the average trade in as £5,175 to £5,500 (or there abouts).

I have 2 questions which i suppose would set my mind at ease. Firstly has anyone else used the vauxhall site and compared prices with Glasses and secondly i assumed that the general glasses applied without adjustment to Northern ireland.

Any help would be appreciated.

Thanks

Colm