possibly you can if virgin has some kind of agreement with croation (did I spell that right?) then yes but normally you can for example if you fly virgin to LHR then BA to Edinborough,Scotland and try to check your baggage through to Scotland you won't be able to (and virgin will start to show a strong dislike twards you them and BA hate eachother)but if you took say CO from anywhere to EWR (newark,NJ) then virgin to LHR of virgin to LHR and BMI to Dublin,ireland the great fine by them. But to be on the safe side (if they can but don't have a codeshare agreement then checking it through most likely voids any ammount of garentee by either airlines that your bags will arive in croatia.) ask the rep at the ticketing counter. If not allow youself atleast 3-4 hours for a layover depending on traffic at heathrow less if your flying first, bussiness or have some way of going through the exppress security line (also check with the state department and the british equivilant to see if you need any extra documentation (a diffrent visa,ect...) as you would be considered a visitor to england for the duration of your connection as transfering between terminals would envolve leaving the terminal and the bus/train would most likely be before security and a visa/pass for a plane transfer may not cover this.

let me know if this helps