If you do everything by yourself, it's 13 GBP per year for the filing. You will need a UK address for the company, which can be had for a few hundred GBP per year.
If you outsource it to an accounting firm, it depends on volume (number of transactions). Can be anywhere from a few hundred to several thousand GBP. Best to ask around for quotes.
Before you start, take some time to read into how tax residence and double taxation might apply where you live. There is a high risk that the UK company is considered tax resident in your home country and unless︀ there is a double taxation treaty between the two countries, you risk paying double tax.︁ If there is a double tax treaty, you would normally pay 19% in the UK︂ and the rest in your home country.