How much is an engine rebuild (UK only)?

I have a prestine Nissan 100 NX, which I LOVE! It's nearly 20 years old but looks and drives like it just rolled off the production line. Now, I know the actual cash value of this car is neglegable, but the sentimental value makes it worth spending money on it to keep it alive. It has 120,000 on the clock and drives like a dream but I'm wondering, when it does start to approach the 200,000 mark how much would it actually cost to refurbish the engine? In pounds sterling only please!