I don't normally post on these boards normally just read what everyone else has to say but I am always interested in talking about tools! You will have to look very hard to find a true tempered stainless steel tool whether it be a spade, fork or hoe, many are pressed steel, welded together and are just shiny tat, ceratinly no one makes a solid tempered stainless steel spade in the UK. I use an early Spear and Jackson spade from the late 1950s made fro heavy stainless stel that is brilliant but I have not found any modern stainless steel spade that has the same weight, strength or that is solid forged. If one was made you would see it in the price therefore carbon steel would be my preference for most tools. I sharpen all my tools with a tool file and then a scythe stone.
I would disagree that proper spades, even some shovels, and forks are just pressed steel, in other words mild steel. They wouldn't last 2 mins and would deform out of shape the first time they were forced against heavy clay or even a submerged brick. The only consolation is that they would deform rather than snap, as mild steel is quite malleable (low carbon) which would be safer. Hoes may well be pressed mild steel. The duty intended doesn't require tool steels. Spades & forks for £2.99 may well be mild steel, but which serious worker would buy those? It's obvious in the price. Though see below.
Decent spades and forks will be made out of carbon or alloy tool steels, forged and then heat treated. The grade used and the treatment will be geared towards Toughness more, or just as much as Hardness, though the 2 properties tend to go together to some degree. I'm not sure what grades are used (now's the time to research what I maybe should already know), but I suspect a silicon-manganese carbon steel would be a good material. This is the grade used for vehicle springs, including leaf-springs. Very tough, fairly hard, will bend without deformation. Can be sucessfully heat-treated.
A tempered stainless steel spade? Stainless steel isn't hardened and tempered in the same way as a carbon steel. The toughest stainless steels are precipitation-hardened, different entirely to heat and quench methods. There are moreover, 3 general classes of stainless - austenitic (commonest and non-magnetic), ferritic (cutlery, knives, surgical instruments and magnetic) and martensitic (toughest, hardest, least corrosion-resistant and magnetic). I'm not sure what my spade is, but a test with a magnet later today will give a clue.
I'm sure one of the major UK tool suppliers will supply a properly forged & hardened stainless spade and fork. It depends how much you wish to pay. The ultimate grade made the ultimate way may well these days be too expensive to justify production as there will be little demand, although in the case of the humble spade, I'm sure this won't be so.
Whilst I agree that older equipment (in anything, including antique furniture) tends to be heavier and better, (principally because labour costs were so low thus allowing a luxury in material choice) this isn't to say that such things aren't possible today. In the case of metals and particularly stainless steel, there are far superior blends and processes available than there were in yesteryear. Some of the super-alloys as fiddled around with in the nuclear industry and by such folk as NASA are very impressive though this would be an area where costs exceeded potential demand. British tools & equipment were the finest around in what was known as the Workshop of the World. The principle aim was fitness for purpose, longevity and ability to maintain. Cost of production in Labour & Materials was a secondary consideration. Due to the modern world (shrinking resources and mainly higher labour costs), these principles have been sadly abandoned. Also, if you make something that does the job and lasts forever, nobody will come back for another purchase!
Disproving though the theory that cheap is always bad, I'll say again that I picked up first a stainless fork, then months later a stainless spade from Aldi and both were less than a tenner. Of course, I studied and hefted them carefully before buying but on the whole, it wasn't just a case of they're worth 8 quid or so, but that they seem they'll do the job. And they have. Aldi have some good stuff - you just have to wait till things come along as they have a 'specials day' every thurs and sunday. I'm there every week, rooting about. Of course, the tools may well not be UK, but German like the company, but no fools themselves the Germans when it comes to engineering.