I can buy a new Onkyo 515 for £149, that means yours is worth no more than £75. The speakers are currently £87 new, so they are worth no more than £45 at best.
Next, we adjust these prices based on how desirable the product is. If it is rare and highly desirable, you might be able to charge a premium, but these are low end products and as such would fetch no more than I have stated and likely less.
Finally, the functional and cosmetic condition of the equipment effects the price. To get my suggested prices, it would have to be in very good condition. Average condition would be less.
I would say, you might be able to get £100 for it, but there are new 5.1 micro systems with speakers and DVD players that are close to that price. Consequently, I would say you might be able to get £75 for the lot.
My standard formula is to divide the current selling price in half, and adjust for market demand, condition, and age. That's half the current actual discount selling price, not the manufacturer's suggested selling price.
Steve/bluewizard