So, basically, the more domains you own, the higher the price you can charge?
Yes and no. Assuming that they are generic domains of sufficient quality that somebody would want to buy them, then I would say yes.
The more quality domains you have, the more you're increasing the chances that somebody somewhere will pay a lot of money for one of them (assuming that you've priced them that way). This leaves you more leeway to experiment and to price your domains higher.
Another way to look at it...
If you charge 1,200 pounds for a domain name (and sell it), that "carries the cost" for about 200 more domains. You need to sell 0.5% of your inventory every 2 years - 0.25% a year - just to break even on renewal costs.
If you charge 2,400 pounds for a domain name (and sell it), that "carries the cost" for about 400 more domains. You need to sell 0.25% of your inventory every 2 years - 0.125% a year - just to break even on renewal costs.
If you charge 10,000 pounds for a domain name (and sell it), that "carries the cost" for about 1,500 more domains. You need to sell 0.067% of your inventory every 2 years - 0.033% a year - just to break even on renewal costs.
Now obviously you'll sell more domains by charging 1,200 pounds than you will by charging 2,400 pounds, and much more than by charging 10,000 pounds per domain.
But will you sell ENOUGH MORE to make it worth your while?
Really, you have to try it and see. Play with the prices and try and find the "sweet spot" where the pace of sales is enough to pay for renewals and maximise your profit.