Often you can just go to the UK and the number will work. You may need to call your mobile provider to enable international roaming (quick phone call, should be a free setting.)
But as you said "Roam", it's likely to incur significant cost (unless you purchase a specific international roaming plan from your mobile provider).
My suggestion is when you get here buy a SIM card that has minutes and texts preloaded (will have a UK number). You can even get a phone for free or as little as £10 with these SIM cards. You might want the phone if your phone is locked.