Relocation is never an easy decision and it involves a lot of planning and lot of patience especially if moving from one country to another.
As you mentioned that you are planning on starting a business and also planning on moving to a different country which would not only have visa requirements but also it might need some time and extra work on your part getting use to the rules and regulations in a new system for not only business operations and start up but also for you getting use to and setting down in a different environment.
As you also mentioned that you might not come back to UK or on the other hand you might have to come back to UK to start the business.
The things to consider are:
1. The line of business that you are planning on starting is that more catered to UK market or is it a business which you have done research and know that you can successfully start and run in the new country with different rules and regulations from UK possibly.
2. Would you get the same potential results in the new country as you would in the UK?
2. Do you have a support system of expertise in the new country who can help you with setting up your business and guiding you in the process.

If you have a good support system and also have enough capacity to survive in a new country starting up new business and overcoming the hurdles which might arise in the process and be patient in the process of a foreign country business regulations then yes by all means so there first and then start your business.
But if there are some hesitations and silent moments then perhaps it would be better to start in UK and expand it.
The thing which needs some attention is that are you looking to migrate to another country for long term then don’t start the business in UK.
If you feel you have all the reason for believing that you will not come back to UK then there is no point in staying and starting in UK provided you have a clear understanding of the rules and laws in the other country and also the business is something that would be a success in the other country then yes by all means go there and start.
If your business is of a service providing nature or an online business then you can start anywhere.