If everyone in the world possessed the knowledge contained in this short article, a couple companies would probably go bankrupt and you might be saving some money.
Lossy Analog, lossless digital
To answer this question properly, it is important to realize the "primary" technical difference between analog and digital data transfer. Simply said - analog signal is lossy, digital is not - at least not from the point of view of regular consumer electronics available. This means that a gold plated analog SCART makes a difference in what you would call "picture transfer quality", but a gold plated digital HDMI does not as the data stream that enters a given HDMI cable reaches the other side completely unaltered. A £20 HDMI cable therefore offers the EXACT SAME "picture quality" transfer as a £90 premium gold plated grade HDMI.
For a regular customer with a LCD/LED, 3DTV or others, a premium HDMI cable can make an impression of offering more visual quality than a regular one based on his former experience with analog SCART and Component connectors. While it is true that a gold plated premium HDMI is superior to a regular one, it is however so in ways that don't matter that much around consumer electronics - probably the most notable one being maximum bandwidth. While a regular HDMI can achieve maximum transfers of about 10 Gbit/s, a premium one can go as far as 17 Gbit/s or even more. Also, such cables are usually more durable.
Taking into account that a regular HD stream on Freeview uses about 20 Mbit/s (eg. about 500x less than a regular HDMI can theoretically transfer), it is quite clear that even a regular HDMI that meets the official HDMI specifications is quite sufficient by itself.
Although there are applications where a premium gold plated HDMI is advantageous or even required (for professional grade electronics), a regular cheap HDMI is able to offer EXACTLY the same picture quality - meaning lossless. The sales staff in a shop might tell you differently, but then again, sales staff in a shop is paid to sell products.