XPS 8300, adding USB 3?

I recommend that you read the reviews and determine if you are OK with the horizontal orientation of the USB connections. Some reviews indicate this is a problem. There are other expansion cards where the USB connections are vertical. 

USB3 cards that are powering things should be AT LEAST X4. There is a reason why that card is sooo cheap. Its junk.

https://www.sonnetstore.com/products/allegro-pro-usb31-pcie

Sonnet Allegro Pro USB 3.1 PCIe Card, 4-Port SuperSpeed + USB 3.1 Gen 2 Card Dual 10Gb

USB3-PRO-4P10-E

Sonnet's Allegro has the following advantages:
Delivers 7.5W of power per USB port for USB-powered devices

Doesn't require separate power plug connection; gets the power required to operate connected drives from the slot it’s installed in Independent, per-port power regulation protects each port from glitches on adjacent ports, preventing auto-disconnects of already-connected drives. Features a re settable fuse on each port, protecting both card and computer should a cable or peripheral fail; resetting a tripped fuse is simple—just shut down and restart your computer

The Sonnet Technologies card you linked to on amazon.co.uk is not the same card as recommended; it is only x1. While @speedstep recommended the x4 card, I would like to point out that according to the specifications in the Setup Guide for the XPS 8300 there are no x4 PCIe slots only one x16 slot and three x1 slots.

PCI-E 2.0 Gen2 is what the XPS 8300 is shipped with Intel H67 Chipset.

That means USB3 bandwidth especially for more than 1 port is not available.

So X1 card would require AUX power connection in order to provide USB3 7.5W power per port aka 30W for 4 ports.

Something Like this

https://www.amazon.com/dp/B00FPIMICA

The card the OP Linked is Junk and NEVER recommended.

In the original manual for my XPS 8300 it mentions "one back panel and one front panel USB 3 compliant connectors (optional). There had been no option to add any USB 3 connectors when I originally purchased the PC from Dell UK. Was this referring to a add on PCI-E card? 

As I only want to connect just one USB 3 device to the PC a single connector would be fine.   

I have found the same card you just recommended on Amazon UK. Could I use the power cable from the PSU which used to power my onboard optical drive unit to power it? No longer use built in optical drive.

https://www.amazon.co.uk/Inateck-KT5001-Ports-Express-Connector/dp/B00FPIMJEW/ref=sr_1_1?dchild=1&keywords=Inateck+PCI-e+to+USB+3.0+%284+Ports%29+PCI+Express+Card+and+15-Pin+Power+Connector%2C+Red+%28KT4001%29&qid=1600603989&sr=8-1

@Fly-Catchers wrote, "In the original manual for my XPS 8300 it mentions "one back panel and one front panel USB 3 compliant connectors (optional). There had been no option to add any USB 3 connectors when I originally purchased the PC from Dell UK. Was this referring to a add on PCI-E card?"

The answer is no it is not referring to an add-on PCIe card. The one front panel USB 3 compliant connection is optional because there is only one USB 3 connection on the motherboard. See #16 on the System Board Components diagram in the Service Manual. If the USB 3 connection is used for the FlexBay slot (supports optional Dell FlexBay compatible device) it would not be available for the USB 3 connector on the front panel. So I think you have to look at your computer and determine whether you have a FlexBay slot and how the USB 3 connection on the motherboard is being used.

@Vic384​ following up 5 years later... Part #16 is the 3rd USB connector, not USB3.