Your home Insurance should cover it for "New for OId" a Travel Policy will take into account wear and tear and as you have discovered has low single limits.
Travel Insurers do not really like paying baggage claims and will normally insist on seeing receipts for the lap top, your home Insurers will not always insist on receipts.
Plus the Travel Insurers will have quite strict limits on whats covered, your home insurance will have less limits and the other advantage is for your £25ish it will also be covered anywhere in the UK as well.
Please don't leave it in your car unless you lock it in a secure concealed boot or the glove compartment otherwise it won't be covered when in inevitably gets stolen.
If you also add on cover for "Unspecified Personal Belongings / Possessions" your bagagge will have similar cover and you can often get a discount of around 15% off the travel insurance as you won't need their bagagge cover