As no extradition treaty is presently in place between the UK and Russia, then quite simply the Russians were not obliged to do so. As there was no legal framework to compel them to comply with the British governments request (note the word request, because that's all the British government could do). Which is unlike this situation where an internationally agreed convention is in place, and the British government is in breach of the convention by not granting physical access to the Skripals.
The Russians did however offer the Met Police physical access to the Litvenyenko suspect in Russia for questioning, see here:
https://www.reuters.com/article/us-b...62631020061205
The problem as ever is that the British government does not hesitate to get on their political high horse and preach about moral standards to all and sundry when it suits their agenda. However when the situation is reversed they are conspicuously quite, ie hypocrites to the core.
Put quite simply granting the Russian access to the Skripals does not suit the agenda the British government is pushing, because to do so would raise a whole new load of awkward questions requiring uncomfortable answers. Which in all likely hood would further undermine the British governments narrative on this whole affair.