In relation to the Exit Re-entry Permit , you have 2 options : single or multiple . The single (SERP) is normally issued for 3 months , but I think it is possible to get it for up to 6 months . the multiple can be for 6 months , one year , and 2 years .
So I think you have 4 options :
1. SERP for 3 or more months ,
2. MERP , 6 months ,
3. Before you leave to UK , cancel the KITAS (ask for an Exit Only Permit/EPO , return KITAS/"blue book" to your Kantor Imigrasi) ,
4. If your SERP , or MERP expires before you return to Indonesia (in this case the KITAS is canceled , but not in a regular way) , you will need a new visa to come back to Indonesia , and , at the airport , you may have to explain why you didn't get an EPO ( maybe a doctor's letter stating your medical treatment can help) . I believe that if you are with your Indonesian spouse (sponsor of the KITAS) , there will be no major problem (I would ask Kantor Imigrasi about that) .
I believe that if you get only a 3 months SERP , when you are in UK it is possible to extend it , if necessary . I guess the extension need to be requested by someone (with a surat kuasa?) at your city's Kantor Imigrasi (?) , or at Direktorat Jenderal Imigrasi in Jakarta (?) . I think that after the extension approval in Indonesia , Immigration will charge Rp50k to send a telex to an Indonesian Embassy in UK (I would ask Kantor Imigrasi about it too).
I guess that a SERP or a MERP will be limited up to 30 days before the expiry date of the KITAS (these 30 days is supposed to be use for the KITAS extension) .