Hey nat88
You don't need to get a tourist visa in advance, but it would be a good idea to have proof that you're going home when you try to re-enter the UK.
Passport officials will most likely be a little suspicious (that's their job after all), but if you can show your flight booking for when you are departing that would be a good idea. It might also be worth your while having a letter from your employer confirming that you have resigned to return to wherever you are from. Any other documents which support this might also be worthwhile (eg freight receipts if you've sent stuff home).
Of course, you might not need any of this but it's certainly worth being over-prepared than the alternative.
Cheers
TNT ed