Hi All, I'm looking to buy two more sourcebooks and would love recommendations. To help with that I currently own the Core Set: (PHB, DMG, MM), Essentials kit, and Creature & NPC card set. So, looking for the next two purchases as I have around £60-70 to spend.

I DM a homebrew world in which there are no firearms and tech is being suppressed by a particular force in the world. Having played with a rather diverse group, I intend to continue disallowing artificers and probably changelings in my current and future games (except for particular circumstances/players) as I feel they don't fit into my world of Eternarii. I will tend to homebrew magical items (benefits usually come with drawbacks, and my players have picked up cursed items previously too). I also tend to be of the opinion that if my players want access to a subclass or particular race it's on them to have those sourcebooks (perhaps unfair, but I can't afford the whole set).

With those things in mind, what sourcebooks would you all recommend? I've been looking at Xanathar's Guide and think that's probably a good start. Where would you go?

Xanthers and Tasha's are the two next best books.

Xanthar's is slightly better for Wizards (more spells), while Tasha's is more desirable for the other classes.

I’d second xanathar’s. It’s pretty universally well-liked.
Personally, I really like Tasha’s, and find it to have a lot of interesting options. But there are a number of people who have issues with it. Most of the problems seem to be a dislike for floating racial ability score modifiers, a belief that some of the subclasses are overpowered, and finding some of the material not useful for experienced DMs. 
Either way, those two are probably the most useful for general, non-setting specific material. 

I would definitely recommend Tasha’s, but checking out Mordenkainen’s Monsters of the Multiverse (AKA the book of more Mordenkainen alliteration) is probably worth.

I agree with everyone else about Xanathar's Guide and Tasha's Cauldron.  I would Include Monsters of the Multiverse since it is the new Races handbook, plus it has some good monsters to add to your adventures.

Quote from BKThomson >>

I agree with everyone else about Xanathar's Guide and Tasha's Cauldron.  I would Include Monsters of the Multiverse since it is the new Races handbook, plus it has some good monsters to add to your adventures.

 Exactly.

I think MoTM is actually most important, because it has the new races and monsters, instead of just archetypes and spells.

Quote from VoidFlam3 >>

I think MoTM is actually most important, because it has the new races and monsters, instead of just archetypes and spells.

 So, as I said above, if it's just about races that's for my players to invest in and bring to the game sessions. I've actually got access to MoTM through Beyond and it being shared by a player somewhere. Given that I have Monster Card sets again it doesn't really sell me on the book that much. 

Appreciate everyone's thoughts though. Have decided to go with XGtE and TCoE.

great choice. XGtE and TCoE are good.
MoTM is literal trash. This generic origin from TCoE is such trash and it just breaks the older races. Guess you can take it for the extra races if you don't plan on checking out the other source books that already contain them.

• MoTM is pretty useful for DMs if you aren't comfortable making your own monsters and you don't have the other monsters books. It will give you good bank of monsters and examples to build your own. It also provides allot of races. 
• TCoE is really better for player options, buy it for your group if they want more stuff to work with or for yourself if you are a player however general ideas like choosing ability scores are pretty easy to come up with so you don't really need it.  There are some optional class abilities which i think are important like the monk ones but they wouldnt be too hard to self implement either. 
• Xanathars is good for down time activities and clarifying some mechanics like how spells work so probably a good option for DMs. You can home brew these things but its allot harder than the stuff in tasha's.