International Living have the following pertinent advice on Mexican taxation for expats:
'As an expat in Mexico, the taxes you’ll pay depend in part on your situation. If you own property in Mexico you’ll have real estate taxes. If you rent out that property or own a business, have a job, or have interest-bearing bank accounts, you’ll owe income tax. Even if you have none of these, you’ll still pay sales tax at shops and value-added tax when you eat out at restaurants.
It’s a good idea to meet with an international tax specialist before you move to Mexico; he or she can advise you on how to best minimize your tax obligations, especially if you have significant assets in both Mexico and your home country. If you do business in Mexico, you’ll also want a good tax advisor there.
Income tax in Mexico:
You will owe income tax in Mexico if you hold a job, run a business, rent out a property you own, or hold an interest-bearing bank account or security in Mexico. In most cases you will need to file a Mexican tax return.'
You could register as self employed for your on line business in the UK using your accommodation address. HMRC will then send you a self assessment tax return to complete annually at the conclusion of each tax year. This return can alternatively be filed on line.
In any event there is a Double Taxation Treaty between the UK and Mexico which precludes the same income stream being taxed in both jurisdictions. This is achieved by means of tax credits, the tax paid in one country being allowed as a credit against any liablity in the other on the same income. The Treaty does not, however, protect you from differences in rates of taxation.
I do hope that you find my reply of assistance.