I use specialists for many of the jobs that require specific kit and expertise. I have several trade accounts with large and small suppliers but I generally like to go to the specialist plant growers for purchases. It does take a little longer in the buying phase, but you get the plants you want rather than the ones that are available.
They specialise in mature trees and shrubs https://www.paramountplants.co.uk
I have no hesitation in recommending Tree Frog Services. They are very professional, easily contactable on text, email and phone and have really invested in the kit needed to get the job done. They also obviously have great contacts for the more esoteric stuff. https://treefrogservices.com
I have had the great pleasure to purchase mature olive trees from this company that is meticulous in making sure that the trees they sell are tested and certified. Their biosecurity measures are beyond reproach and the trees are stunning. https://villaggioverde.co.uk
If I have a bulk buy of perennials, Howard Nurseries is often the place I start because of their range and knowledge. https://www.howardnurseries.co.uk
An absolute gem of a specialist who grows an amazing range of herbs that I use for cooking as well as their properties as shady perennials suitable for our changeable climate.
For mature trees there’s no better than Barcham’s. So good they even have two royal warrants.