Hi Shredder,
I'll try and answer all your questions
Santander Bank Accounts - Are fully supported. I have clients using Xero with Santander at present.
CRM/Invoicing - I'm not sure Xero has CRM as far as i'm concerned. It has some integrations for CRM but doesn't offer it as standard. No need to use the invoicing but I would need to understand how you would manage this better outside Xero. Surely being able to email invoices and check who owes you what is a good thing? I will however concede that you will know your business better than me
Year End - Xero is essentially dynamic in that you can run the P&L etc from any start date to any end date. You also have the ability to lock periods etc. It's not anywhere near as strict an environment as Sage.
Certified Partner - I'm not going to force you down a road you don't have to take. There are ways of restricting access and it does warn you about doing certain things in the system. If you were the only user though you would have full access to everything. Ultimately I think you have to see the value in working with a certified partner. Xero support is excellent so you may be able to get by without one.
Cashbook/Full Accounts - It does have a cash book it can't as far as i'm aware currently produce full accounts. It's being developed and in some countries you can use it to do that. For me though I think the end of year accounts still needs to go via something like VT or Iris. You could use Companies House if really simple which is free other than filing fees.
More than one company - Xero is company based so as much as you can have as many users as you need for each company you do need a separate license for each company.
Ltd or LLP - I dont see any issue with it working with either. We have it working for both these types of client at present.
Payroll - This is seperate but there are integration partners that will pull some data directly into Xero. MyPAYE is probably the best UK one in my opinion.
Backups and csv's - These aren't a problem you can do these easily. It has a reporting centre where you can PDF reports etc and save them for the future. It also integrates with Google Docs for this part as well.
Vs Sage basic - The direct bank feed is the biggest things for me and that will save the most amount of time for most users.
Hopefully i've answered everything you have asked but if you need me to clarify anything I am more than happy to do so