This is brilliant progress. This Boston is your benchmark for now. You really need to try lots of pianos, without buying, and see what sounds great and feels great, irrespective of cost. This means both really cheap and really expensive.
Then when something cheap comes up that sounds great and feels great, it is a prime candidate. I don't know where you are in the UK. But just as an example, there is a Broadwood baby grand on ebay, 1900s, very clean modern looking, in a nice house in Scotland, £100, 1 bid, 1 day 12 hours to go. That might be a fantastic piano. You want something relatively modern, seen light use only, owner moving house or passed away, etc. Something almost as good as new, it has to sound right, and have a good touch like the Boston, which might be a similar quality instrument. Everyone goes to a dealer looking for a bargain, if a decent piano comes in, unless it is over priced, it will often be sold before you see it. It is easy for dealers to get left with a collection of poor old grands. There are poorly restored Bechsteins, Boesendorfers and Steinways out there too, with silly prices on them.
If you want something that plays and feels as good or better than this Boston, in budget, I think you need to be prepared to stick with the digital for up to a year. You need to be totally blown away, completely shocked by the piano you are getting.