Current equity: 125k
Buy: 385k
Savings: 0?
New mortgage: 260k
You've over 20% deposit, and borrowing less than 3 times LTI, so no issues from CBI rules.
Monthly tax home: c 5,500 between you?
Mortgage repayments at 3%: 1,235
stressed at 5% over 25 years: 1,520
You'll comfortable meet affordability criteria, but might lack on evidence if you haven't been saving regularly (they will add current mortgage and the loan repayment though).
Now: where to get mortgage. You can keep UB tracker, but they add 1%. So you can have 179k at ECB + 2.25% (guaranteed for remainder of original term), and fix the rest at 2.6% for 4 years.
You should try UB first to hold onto the tracker. They'll possibly also allow a longer term as you're self employed. It doesn't matter that your bank accounts are elsewhere - you'll just have to get all your statements together - personal and business. They'll just need 6 months worth instead of 3.
Being self employed requires some additional paperwork, but nothing onerous if everything is up to date (accounts and tax affairs). UB have a list of required docs, but treat it as a starting point. They will generally need more!