Where to start?
There are so many variables, and you'd have to research each pub individually. When you say a 20-30% yield, do you mean on capital invested or net profit on turnover? If it's on capital, I think that you will struggle.
You also refer to a well run pub - have you ever run one before? Do you mean to run it yourself or put in a manager? If the latter, then imo £300k turnover is way too low to do this. I ask this because you refer to only wanting to work 2-3 days a week later in the post. Running your own pub is more like working 8 days a week
The variables include the gross profit rate - it will be higher with food, but you'll have to employ a chef, and keep him/her. It will also be higher in some parts of the country than others.
Then costs will vary widely, with the biggest cost being wages, and that of course depends how much you want to do yourself.
How stable is the turnover? What competition is there, drinking and eating? What is the catchment area?
Yes there is money to be made running a pub, particularly if you are free of ties from the brewers or rapacious landlords. But there are more and more going out of business every week. It doesn't take much to tip the balance - upset the current locals and they up sticks to the next pub, serve a bad meal, or a slow meal, and 30 people hear about it.
If you've never run a pub, think long and hard before investing your life savings, and remember that many landlords say that a pub gave them the two happiest days of their lives - the day they opened it and the day they closed it.