So telling the police you have a script may not mean that you get help and if it means that they realise you're a drug user they might change their attitude towards you.
If they confiscated your methadone on arrest they should give it back on release.
Ask your local drug agency what the policy of your local force is and, if necessary, what they can do to change it.
Prison guidelines do allow for longer-term prescribing which is done regularly in some progressive prisons. Tell the prison medical service at the reception interview that you are on methadone and they may continue the treatment. You should at least get a seven-day detox but, if you are pregnant, seriously ill or are on a maintenance prescription and are likely to be released soon, you should receive methadone as you would in the community.