Is it better to start a business before moving abroad, or after?

This depends on the nature of your business, the requirements on how to develop it and on the locations involved.

I'm not aware of your case, but I'd suggest that you are starting in a familiar environment (UK?) and then work remotely from anywhere in the world. This means lesser risks (language, regulatory framework, support...). Of course working remotely becomes rather challenging if you are running a "nuts & bolts" operations like a restaurant or a logistics company.

You can then establish a presence outside the UK in a 2nd step, to expand your network but also to show commitment to that new market. Furthermore: depending on where you go, having a "home base" in the UK can be leveraged as an advantage.

Before start any business , you have to clear project plan and implementation process.
Developing a business plan can help ensure that you’re sprinting down the right path
It will help you avoid big mista
It will help counterbalance emotionskes
It will make sure everyone’s on the same page
It will help you develop a game plan.
It will help you raise capital
Most important TIME and continuous involvement.
As my suggestion you have to dedicate on your business to be successful.
Thats why better to start after came back in UK , start from your root , spread across globe.

The first thing I would ask you is do you have the idea written down? Is your business plan clear to you and anyone you'd offer your service to? Do you have your goals and vision written down for your business? These are the first steps to consider. Once touce done all of that which is part of the business building process and also work. Begin to see if the business you have in mind firs the environment that your moving into. Is the city or nation your in a place that is conducive to business start up and development? What are the benefits of doing business in this place?
With the question your asking its important to think in the long term where are you going to end up, be settled and stable in?. Stability is key, there are ways that you can do part of your business wherever you are take baby steps and even earn a little income from it. For example being a consultant for your area of expertise, giving people advice for a fee. Then when it's time to move you can have some money in your pocket as well as experience. And once your settled work on the over all vision of your business. There is a lot to this question for further details on what are small steps you can take in your transition process. Feel free to reach out to me or contact me. Because there is a lot you can do where you are. But it's very detailed and tailored to your specific business. and it depends what your business is. Hope this helps

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.

Well first, what is your current financial state? Are you secure enough to be able to relocate, acquire and residence and start a new business else where? Or will the potential profit from the new business help you relocate? Also, you should figure out where you want to move, be sure about it, and make certain that its a place where your future business is needed and can thrive...