Absolutely no reason to buy a Windows based laptop except to ensure compatability with health service systems. I assume that the systems administrator will insist on installing some additional security software on your system before allowing it to be connected to health service or other systems. Having a Windows based machine should allow your computer to be more easily integrated into the existing network and allow you to access databases and other information.
On a personal note, I would keep a back up copy of all your research and source data on the iPad or another non Windows based system.
Any modern brand name laptop with an Intel Core 5 or better processor will be good enough. The more money you spend, the bigger screen and the more storage capacity you will have. If you need to do presentations, then having 16Gb of RAM is a good idea to keep things moving along video wise.
Try to keep a grandfather, father, son, backup system using USB sticks for vital information that you can't afford to loose. You cannot have too many backups.
Anything from £500 upwards should be more than sufficient for your needs. Worth budgeting for a wireless printer and a Logitech ball mouse (also colloquially known as a rat). These are a lot easier to use than standard mice or trackpads. A decent full size external keyboard is also useful when having to do lots of typing. The printer, keyboard and mouse can be left wherever you normally do the most work and just the laptop carried about when necessary.
Finally, get a decent carry case or messenger bag for the laptop. Something with protective padding and a decent shoulder strap that allows you to keep both hands free if necessary.
You haven't given a budget but £500 will get you all the computer you need. A decent carry case might cost £50. Mouse and keyboard £100. Wireless printer £100. USB sticks or external hard drive back up system £50-£100.
With careful shopping around, the basic minimum of computer, carry case and back up system could be purchased for £500.
Secondhand equipment will be much cheaper, but you don't get a guarantee or support if things go wrong.