Perhaps the lenders are concerned that you would not be able to service any more debt, even though the debt you've already got appears to be modest in relation to the total value of the assets. This view might be reinforced by the fact that you have credit card debt - i.e. are you spending more than you're earning?
The point about knocking the two properties into one is nonsense in my view. If all lenders were afraid of that then nobody with a semi could get a mortgage. You will almost certainly be in breach of your mortgage conditions if you make any significant alteration to either property without the consent of the respective lender.
The lender attitude to the tenancy point is more difficult to understand, although most lenders would prefer an Assured Shorthold Tenancy, which protects the tenant whilst they pay the rent but also gives the landlord (and therefore a mortgage lender) power to take vacant possession (eventually) and sell the property. Any property's marketability is affected by the terms of any lease that's in place, and lenders will always be wary of anything that's not in 'standard' form.
Regulated mortgage lenders now have a duty to assess 'affordability' before granting loans to individuals, and my suspicion is from what you've said that even though you're asset rich, you might be perceived as cash poor and therefore unable to pass their affordability criteria. I'd expect them to tell you that, though, rather than any bull about knocking the walls down.