Getting an english phone number is one of the essential things you will need when you arrive to the UK. Even tough the roaming has allowed to use your SIM card from any country, however, I would recommend you to have an english number if you are planning to live and work here for a while. Some companies refuse candidates that do not have an english SIM card because the fact of making a call to you can cost them money, therefore I would recommend you to get yourself an english number (remember that you can keep your current number for some apps, such as Whatsapp).
In the UK there are several companies that provide cheap services and plans so, it would depend on the use that are you planning to have of your phone.
Based on my experience, I had first a Giffgaff sim card as this is the cheapest company that provides this services. However, in some places the signal is not very strong so that motivated me to get another company. Giffgaff sim card can also be sent to you by post, therefore, you can receive your number before you move and you can start apply for jobs while you are still in your home country.
Other companies that operate in the UK are: