Everybody thinks Buy-to-Let is a receipe for making money quickly but there are a lot of downsides, especially if you are an inexperienced landlord. For a start, you will have your mortgage payments every month and possibly periods when you have no tenants either. And when you do have tenants, they may cause damage or not pay their rent. You will also have hassles if equipment breaks down, the plumbing leaks or the place needs redecorating. Are you prepared for all this? As well as having to keep accurate accounts of all your expenditure?
Do you have an adequate pension? That perhaps should be a priority.
Possibly a good mix of unit trust funds would cause you less hassle, rolling over your maximum �7,000 ISA allowance every year for as long as they last. Then just reinvest the dividends. If you are a taxpayer, any long term capital gains in ISAs are tax free. Profits on Buy to let properties, if you are lucky enough to make any, are taxable.