Trochę mało kontekstu, ale z tego, co podane (lenders), wnioskuję, że chodzi o oprocentowanie kredytów zaciągniętych w celu kupna nieruchomości na wynajem.

Buy-to-let is a British phrase referring to the purchase of a property specifically to let out, that is to rent it out. A buy-to-let mortgage is a mortgage loan specifically designed for this purpose. Buy-to-let properties are usually residential but the term also encompasses student property investments and hotel room investments.


--------------------------------------------------
Note added at 18 mins (2018-11-10 21:33:32 GMT)
--------------------------------------------------

http://alphdupre25.blogspot.com/2015/04/an-overview-of-buy-t...

Although recently there was a shortage of high loan to value mortgages, there are actually some lenders offering 85% LTV on buy to let again making it a more affordable option for more people. In February of this year the Telegraph ran an article with the following headline, “Buy-to-let 'even more attractive' as mortgage rates hit record lows” stating that lenders are reducing buy to let rates and relaxing lending terms. In fact the number of buy to let mortgages available is reported to be at 800 now – up 100 in 3 months.