There are 2 things that need to be taken into consideration here.
Firstly, as many have pointed out, the rate of 1.99% is extremely good and it is unlikely that you will find a comparative rate on the market. Saying that with a balance of only £18,000 the interest rate is not going to have much effect on the payment.
The other thing to consider is the term over which the loan is taken. Your mortgage is likely to have a longer term than the personal loan and this means that you will be paying interest on your borrowing for a far longer period of time and while it may be at a far lower rate it will still add up over the term and most likely be a more expensive option even if the monthly payments will be lower
If the current mortgage is calculated over 10 years it means the payment would be around £165 per month and the total repayment would be around £20,000 (give or take a few hundred asnd assuming no change in rates over this whole term).
If you assume a reasonable current market rate of around 3% and consolidate the loan into the mortgage and take it over the same 10 year term the payments rise to around £220 and the total amount payable rises to around £26,000.
If you left the personal loan to run its course over a period of say 5 years then you have higher repayments of about £85 per month but the total cost of repayment is just over £5,000 which works out around £1000 cheaper than consolidating the loans into one. The repayments may be higher for the personal loan but because it is over a far shorter period the amount of interest that you will pay back is smaller than on the consolidated loan.
Just an assumption on the terms, if you have similar terms for loan and perosnal laon the it may just be worth consolidating but on basic assumed common facts it would most likely be best to stay exactly as you are.