The current standard "SelectPC" we (the School) buy for Windows or Linux is an "HP Elite 800" business desktop. The price is typically about £500.
Generally we tend to buy "small form factor" desktops these days. Our standard configuration for 2015 is a quad-core CPU, 8GB RAM, 250GB hard drive (we generally don't want you to store your important files on your PC, so we don't need very large hard drives) and a 23" widescreen display, typically at "full HD" (1920x1080) resolution. Staff and PhDs should generally have a PC provided for them by the School. As a PhD student, if you receive a new machine when you start with the School it will generally see you through to the end of your degree.
If you have particularly unusual computing requirements you may instead opt to buy something else using research funds, e.g. from a grant or from your "start up" funds or RTSG. You should always discuss such alternative purchases with the IT team ahead of time. If you can convince the IT staff of your "case" we will make a contribution towards the cost of alternative equipment in lieu of providing you with a desktop PC. For academic staff this contribution is up to a maximum of £900, £750 for PDRAs and for PhDs we will contribute £500.
If you spend most of your time away from your desk it may make sense for you to have a portable PC as your main/sole PC. The University has a "Select Notebook" scheme to try and standardise on a relatively small number of machines so as to make support and procurement as straightforward as possible. A typical "Select Notebook" will have a dual core CPU and 4GB RAM, with a 250GB hard drive or 128GB SSD, DVD/RW optical drive, and a battery that should last several hours of light use. Screen sizes are typically 13" or 14". The "standard" screen resolution for most notebooks is currently 1366x768 - we usually recommend an upgrade where possible to 1600x900 or so. You may also wish to upgrade to a discrete graphics chip, higher capacity battery, etc. A basic laptop (we're currently tending to recommend the Lenovo Thinkpad X240) should be about £610. Upgrades to hardware (e.g. SSD in place of a mechanical hard drive) or warranty/insurance cover (e.g. 3 years' accidental damage/theft insurance) will add to that.
We will insist on the addition of a "desktop kit" consisting of a desktop screen, keyboard and mouse and a docking station if you have a notebook as your main PC. This will typically add at least £225 to the cost (dependent on the screen you choose). A fairly highly specified notebook that is intended to be a user's main/only PC will probably cost in the region of £1200, possibly more. The "desktop kit" is NOT OPTIONAL - we've had too many cases of people developing RSI/posture type problems by trying to use a notebook as their main PC, so it is a Health & Safety requirement that a desktop kit should be used in such cases.
In each case faster processors and/or more RAM or hard drive space are available at additional cost, however these are the minimum specifications we would be happy to recommend, and should provide a good standard of PC suitable for the vast majority of members of the School.
Staff and PhDs may use Apple Macintosh desktops and/or portables, but we recommend against it quite strongly - we are not able to give you as much support as we would like, and occasionally Apple manage to break access to one or more of our services (printing, file storage, etc.). In the last few years, it has been known for it to take weeks or even months for such issues to be resolved by Apple. If you absolutely must use some software that is only available for MacOS then it may make sense for you to buy a Mac, but if you mostly use general productivity applications a PC makes a lot more sense. Generally Macs tend to be quite a bit more expensive than the roughly-equivalent PC we would recommend. Also note that Apple notebooks are not "dockable" in the same way as business notebook PCs. You will either end up with a "kludge", or a very expensive solution (i.e. one of Apple's "Thunderbolt" displays) if you want to use an Apple notebook as your main machine.