If I transfer my UK pension into a QROPS will I have to buy an annuity?

No, although you may do so if you wish. UK law does not compel you to buy an annuity either but if you do not do so there will be a 55% tax charge on the fund following your death after benefit has been taken.
Having your pension fund in a QROPS means you can keep your fund invested in real assets and gain the advantage of passing any remaining funds upon death to your nominated beneficiaries free from UK taxation.