When I physically transfer the laptop to the buyer does this make them the legal owner of that item?
In general, when you complete a contract of sale, the goods have moved from being the seller's property to being the buyer's property.
The exact point in time when this occurs depends on the contract and the circumstances and, unless something unforeseen happens to the property right about that time, pinpointing the exact instant is rarely necessary.
For example, if I buy eggs from the supermarket, while they on the shelf or in the trolly they are the supermarket's eggs, once I've paid for them and I'm carrying them out of the store then they're my eggs. However, the exact instant when they became mine is hard to define. Was it when they were scanned, when I picked them up afterward, when I paid for them, or when I ate them? Normally, it doesn't matter. However, if the goods are damaged or destroyed without fault on either party, who owned it when that happened matters.
Usually, risk in the property (i.e. being responsible for damage done to and by the property) passes at the same time as ownership of the property but it doesn't have to. It's common to be at a different time in international shipping: for example, ownership may pass when loaded on a truck in the vendor's yard in Spain but the risk doesn't pass until it hits the wharf in Brazil - that is, the buyer owns the goods while they cross the Atlantic but the seller pays for a replacement if the ship sinks. Another example is in building, once something becomes a fixture on land it belongs to the landholder (under English/Welsh but not Scottish law) but most construction contracts assign the risk of loss to the builder until the works are handed over.
It can also matter in the event of a dispute. If you have transferred ownership of the laptop then if you refuse to hand it over then that's stealing - even if the buyer hasn't yet paid or paid in full. Your legal obligation (in most circumstances) is to hand them their property and sue for the debt.
Written contracts are usually quite explicit when the transfer happens. For example, when buying real estate the contract will usually specify a date and time when settlement occurs. For example, noon on 9 October 2020. Even though money might change hands (electronically) at 11:55:32 or 12:03:18 the ownership is transferred precisely at noon. That means that if a fire breaks out before noon then the vendor (or their insurer) have to cover the loss; on or after noon, then the buyer does.
Are there sections of UK Law that actually cover this?
Yes. Basically, all of contract and property law covers this