Yes, car leasing can help to improve your credit score. Just like any type of monthly payment like your mobile phone contract, car insurance and credit cards can all help to improve your credit score as well. 35% of your credit score is based on your payment history, this means car leasing is a great way to improve it. However, you must make sure that you're able to meet the payments each month. If you miss any of the payments, this would have a negative effect on your credit score.
You should try to avoid spending more than 20% of your monthly income on a car. The average salary in the UK is 30k per year, after-tax monthly take home will be £2,000, this means if you're hitting the 30k average, it's a good idea to keep your monthly payments at £400 or less.
If you want to be extra frugal you may want to keep your monthly payments between 10% - 15%. It's important to keep in mind all of the other payments which come with leasing a car such as insurance, fuel costs and maintenance costs if your lease doesn't include maintenance.