fails to cock it might be worth investigating why before attempting to sell it. In non-working condition, with no explanation as to why, it's obviously worth a lot less than in working condition... It might also be worth buying the missing bits from John Knibbs and selling it "complete".
Has the Britannia been rubbed down to bare metal? If it has I suppose this will knock quite a bit off its value. They seem to sell privately for between about £300 and £500, depending on condition.
To be honest, without plenty of decent, detailed, sharp, well-lit pics that get across the precise condition of these two rifles, you'll struggle to get an accurate valuation. It might be worth buying/borrowing a decent camera and taking a number of pics outdoors on a fine day...!
The Vintage Diana Forum
The Vintage BSA Forum