The line between a good boutique bed and breakfast, and a small luxury hotel is very thin these days, and often the difference comes down to a marketing decision, rather than any substantive difference. Boutique hotels tend to be more common in the major cities of the UK, whereas boutique B&Bs tend to be more common in the countryside. The other noticeable difference is that in a B&B, breakfast will be included in the rate, whereas it may be extra or only included in some types of rate, at a boutique hotel.
However, there are no real reasons to get hung up on something that is only a matter of terminology. In general, it’s best to spend a little bit of time reading the online reviews of the place which you’re considering, making sure that it meets all of the requirements that you have, and that it’s a reasonable price compared to other options in the area before you book. Whether it describes itself as a B&B or a hotel will make precious little difference to the quality of your stay. So long as you do your homework, you’re in for a great time.