Think about revenue and profit multiples as a gauge of what the scale of business is. Although it varies from industry to industry (and depending upon the types of assets held, success and untapped potential etc.) as a rule of thumb 2-3 times revenues as a sales price, or 6-10 times earnings, before interest, tax, depreciation and amortisation (profit).
That would give you a gauge. So if you are looking to raise a couple of million plus, 800k seems likely, if not, they would be giving it away for a song and therefore it is quite unlikely to be at that level. No idea what EBITDA margins are typical for optometrists but assume 30% (which is typical in many indsutries), would suggest £255k profit on 850 k turnover. Take a generic multiple of 8 times earnings again would suggest a value for the business in excess of £2m. If you are not being asked that, it is very unlikely that there is a turnover of 800k+ sloshing around for you.
Sounds like you haven't done very much homework yet, i would slow down and do some real due diligence rergarding the existing business' performance and financing for the purchase.